Dioon sonorense

Dioon sonorense is a small cycad from north-western Mexico, growing in southern Sonora and northern Sinaloa, in dry tropical forest, oak woodland and xerophytic scrub on the foothills of the Sierra Madre Occidental. Long known as Dioon tomasellii var. sonorense, this species of the genus Dioon was for twenty years the northernmost Dioon: since 2018, its northernmost populations have formed a separate species, Dioon vovidesii. It has a short trunk, often barely above ground, and upright leaves with very narrow, slightly sickle-shaped leaflets. Listed as Endangered by the IUCN, it is very rare in cultivation, and its reputation for hardiness rests on no published measurement.

Key facts about Dioon sonorense

  • Accepted name: Dioon sonorense (De Luca, Sabato & Vázq.Torres) Chemnick, T.J.Greg. & Salas-Mor., published in Phytologia 83(1), dated 1997 and issued in 1998; accepted by The World List of Cycads (version 2026.08.15), GBIF, Species+ and Kew’s World Checklist of Vascular Plants, the database behind POWO (June 2026 version, published on GBIF)
  • Former names and synonyms: Dioon tomasellii var. sonorense De Luca, Sabato & Vázq.Torres (1984), the basionym; Dioon edule var. sonorense (De Luca, Sabato & Vázq.Torres) McVaugh & Pérez de la Rosa (1992)
  • Family: Zamiaceae, order Cycadales
  • Common names: no English name attested; palma de la virgen, peine and peyote in Mexico
  • Natural range: Mexico, southern Sonora and northern Sinaloa; 1,000 to 1,800 m according to the 2020 revision, with lower collections
  • IUCN status: Endangered (EN A2cd+4cd), assessment by J.D. Bösenberg dated June 2019, published in 2022
  • CITES and other protection: CITES Appendix II; Annex B in the European Union; “in danger of extinction” (P) under the Mexican standard NOM-059-SEMARNAT-2010, category kept in its 2019 update
  • Documented hardiness: no measurement or dated report; a European nursery rates it zone 9b, with possible damage below 0 °C (32 °F) (see Cold hardiness)
  • Light and soil: not documented outside nursery advice; in the wild, dry, rocky slopes
  • Habit and adult size: plant 30 to 60 cm tall; trunk 22 to 25 cm across; leaves 80 to 120 cm long (2020 revision)
  • In the UK, US and Australia: Montgomery Botanical Center (five accessions, three from 2004), Fairchild, the Huntington, the Arizona-Sonora Desert Museum and six other American gardens (BGCI); no British or Australian garden listed
  • Toxicity: no data for the species; to be treated as toxic, like all cycads (see Toxicity and precautions)

How to recognise Dioon sonorense

Dioon sonorense is recognised by its small size, its numerous upright leaves and its very narrow leaflets, less than 7 mm wide, glaucous, slightly curved and well spaced along the rachis. The 1984 description separated it from the typical form of Dioon tomasellii by its glaucous, narrower leaflets, 4.5 to 6.5 mm wide, and by an almost cylindrical petiole and rachis.

The revision of the genus published in 2020 by Hernández-Tapia, Jiménez-Ramírez and Vovides gives:

  • Habit: plant 30 to 60 cm tall; trunk 22 to 25 cm in diameter;
  • Cataphylls: triangular with a long point (aristate), woolly, 8 to 16 cm;
  • Leaves: 8 to 20 in the crown, 80 to 120 cm by 20 to 25 cm, felted, sometimes becoming hairless, with light brown hairs; petiole 8 to 16.5 cm, woolly at the base; rachis straight to twisted towards the tip;
  • Leaflets: 70 to 90 pairs, sickle-shaped, acuminate, spreading to drooping, not overlapping; median leaflets 7.6 to 14.6 cm long; margin entire, sometimes with one to three small spines; 8 to 14 veins;
  • Male cones: cylindrical, 25 to 26.5 cm by 7 to 8 cm, almost stalkless;
  • Female cones: cylindrical, about 21 by 15 cm, almost stalkless; scales flat, oblong, trowel-shaped, woolly on the outer face;
  • Seeds: globose, about 2 cm, with a yellow to yellow-orange fleshy coat.

Chemnick, Gregory and Salas-Morales (1997, issued 1998) add characters visible on cultivated plants: a crown of numerous short, upright leaves, sometimes spirally twisted; a slender rachis and petiole, densely downy when emerging and then remaining green; leaflets often armed with one to three small spines near the tip, spaced about their own width apart; and pale green young leaves, barely felted, with a silvery down on the rachis. Those authors, however, described the species in its old sense, which still included the populations now assigned to Dioon vovidesii.

Hybrids

No natural or garden hybrid of Dioon sonorense has been described. In the genomic study by Gutiérrez-Ortega and colleagues (2021), the best-fitting model includes no gene flow between Dioon sonorense and Dioon vovidesii, despite the absence of a clear geographical barrier between the two species.

Possible confusion

With Dioon vovidesii

This is the most likely confusion, since the two species were one until 2018, and many plants sold as Dioon sonorense have no precise provenance. Dioon vovidesii grows further north, in central Sonora. According to the 2020 revision, it is taller (up to 1.5 m), with shorter, broader leaflets, deltoid female cone scales and ovoid seeds; Dioon sonorense has longer, finer leaflets, aristate cataphylls, trowel-shaped female cone scales and globose seeds. The two species also differ in the anatomy of the leaflet epidermis and in their genetics (Gutiérrez-Ortega and colleagues, 2018a). Without a label of origin, a young plant remains hard to assign.

With Dioon tomasellii

Dioon tomasellii, further south (Durango, Jalisco, Nayarit, Sinaloa), is the species from which Dioon sonorense was split. According to Chemnick and colleagues, it has fewer, longer, arching leaves, a thicker and very felted rachis, and broader, dark green, drooping, sickle-shaped leaflets, almost spineless, keeping a persistent down underneath and very close together; its young leaves emerge covered in a thick golden-brown down. The leaflets of Dioon sonorense are narrower, glaucous, often spiny and spaced, and its young leaves are pale green. The 1984 description gave 7 to 12 mm as the leaflet width of the typical variety, against 4.5 to 6.5 mm for var. sonorense. The two species are close relatives: a chloroplast DNA study found in Dioon sonorense a haplotype shared with Dioon tomasellii (Gutiérrez-Ortega and colleagues, 2014). Dioon stevensonii, from Guerrero and Michoacán, formerly included in Dioon tomasellii, differs in the same way by its broader leaflets.

With Dioon edule and Dioon angustifolium

The species was for a time attached to Dioon edule (McVaugh and Pérez de la Rosa, 1992), but that species grows in eastern Mexico and has flat, entire leaflets inserted at right angles. The 1984 authors noted that the northernmost populations of both Dioon edule and Dioon tomasellii have narrow, glaucous leaflets; a plant of Dioon angustifolium, from north-eastern Mexico, may therefore look like Dioon sonorense in cultivation, although the two belong to different lineages of the genus. The slightly curved, spiny leaflets of Dioon sonorense and the provenance separate them.

Taxonomy

The current name is the result of three steps. In 1984, Paolo De Luca, Sergio Sabato and Mario Vázquez Torres described in Brittonia (36(3): 223–227) a new species from western Mexico, Dion tomasellii (the spelling they then used), with two varieties: the typical variety, from Guerrero to Durango, and var. sonorense, from central Sonora to northern Sinaloa, with narrower, glaucous leaflets. According to the 2020 revision, the type of the variety was collected at Álamos (Sonora), at about 950 m (Vázquez Torres 2308). In 1992, McVaugh and Pérez de la Rosa transferred it to Dioon edule in the Flora Novo-Galiciana. Finally, Jeffrey Chemnick, Timothy Gregory and Silvia Salas-Morales raised it to species rank in Phytologia (83(1): 1–6), in an issue dated July 1997 but issued in 1998 according to The World List of Cycads. They relied on morphological characters maintained in cultivation, the absence of intermediates with Dioon tomasellii, different habitats, and a chloroplast DNA analysis by Moretti and colleagues (1993), which found ten differences between the two taxa, more than between several pairs of recognised species.

The content of the species changed in 2018. Gutiérrez-Ortega and colleagues showed, from leaf morphology, the anatomy of the cuticle and epidermis and population genetics, that Dioon sonorense combined two biological units: the populations of southern Sonora and northern Sinaloa, which keep the name, and the northernmost populations, described as Dioon vovidesii (Phytotaxa, 2018a). A genetic study the same year had identified them as distinct evolutionary units (Conservation Genetics, 2018b). In 2021, a study based on 809 genomic markers confirmed the split, placed the boundary near the 28th parallel north, roughly the northern limit of the tropical forests of western Mexico, and estimated the divergence at about 829,000 years (Gutiérrez-Ortega and colleagues, 2021). For the reader, this means that data published before 2018 on “Dioon sonorense”, including cultivation reports and altitudes, may concern either species.

In the 2020 revision, which follows the phylogeny of Dorsey and colleagues (2018), Dioon sonorense belongs to the western clade of the genus, with Dioon vovidesii, Dioon tomasellii and Dioon stevensonii. The revision regards the name Dioon edule var. gentryi, cited as a synonym, as invalid for want of publication; the 2019 update of the Mexican standard NOM-059 still lists “Dioon edule var. gentry” among its synonyms.

Etymology and history

The epithet sonorense means “of Sonora”, the Mexican state where the taxon was described (The World List of Cycads, after Haynes, 2022). The name of the genus comes from the Greek dis, “two”, and ôon, “egg”, for the two seeds borne by each scale of the female cone.

The Dioon of Sonora were known long before 1984: the original description cites collections by Howard Scott Gentry in the 1930s and 1940s, and recalls that plants from the region had been wrongly referred to Dioon purpusii by Paul C. Standley (1920) or filed by Joseph N. Rose under a name never published. The history of the species in cultivation is linked to Naples: the authors of 1984 illustrate one-year-old seedlings, raised from seed collected in Sonora, grown at the Naples botanic garden. A 1998 photograph by Geoff Stein, reproduced by the Virtual Cycad Encyclopedia of the Palm and Cycad Societies of Florida, shows a plant grown in a private garden in Los Angeles.

Common names

No English common name is attested; “Sonoran cycad”, seen online, is not confirmed by a reference source. The 1984 description gives “palma de la virgen” for southern Sonora and “peine” (comb) for central Sonora. The 2020 revision, after Bonta and Osborne (2007), cites palma de la virgen, peine and peyote, and the 2010 list of the Mexican standard NOM-059-SEMARNAT-2010 palma de la Virgen (Sonora, Sinaloa) and peyote (Sonora); its 2019 update gives only “palma”, and Species+ keeps palma de la virgen. These names are ambiguous: palma de la virgen is also used for Dioon tomasellii and other Dioon, and names recorded in central Sonora before 2018 may refer to Dioon vovidesii.

In the wild

Dioon sonorense grows in southern Sonora and northern Sinaloa, on the Pacific slope of the Sierra Madre Occidental. The 2021 study sampled its populations at Álamos and Choquincahui, and the 2020 revision cites collections from Álamos and Choix (Sinaloa); precise coordinates are not published, to discourage collecting. The revision places the species in xerophytic scrub, oak woodland and tropical deciduous forest, and in the transitions between them, at 1,000 to 1,800 m, under semi-arid climates; several of the specimens it cites were nevertheless collected lower, down to about 300 m. The plants collected in Chihuahua are cultivated ones.

Gutiérrez-Ortega and colleagues (2021) specify that Dioon sonorense lives in seasonally dry forests of tropical origin, whereas Dioon vovidesii, further north, occupies xerophytic scrub and oak woodland. The climate of its localities is warmer, less seasonal and wetter than that of Dioon vovidesii, whose winters are colder. The figures often quoted for “the northernmost species”, 615 to 1,200 m and 250 to 500 mm of rain a year on very dry, steep slopes (Chemnick and colleagues, 1997), described the species in its old sense, mainly in central Sonora: they apply largely to what is now Dioon vovidesii.

Conservation status and regulation

Dioon sonorense is listed as Endangered (EN) on the IUCN Red List under criteria A2cd+4cd: a population reduction of at least 50% over three generations, past and continuing, due to habitat degradation and exploitation. The assessment, by J.D. Bösenberg, is dated 28 June 2019 and was published in 2022 (The World List of Cycads, GBIF, Crossref); the previous assessment, of 2010, was by J. Chemnick and T. Gregory. According to the 2020 revision, the making of alcohol from the trunk (sotol, bacanora) has sharply reduced the populations, to the brink of extinction; Bonta and colleagues (2019) write that in Sonora the extraction of starch from Dioon trunks for sotol has wiped out whole populations. In 2014, Gutiérrez-Ortega and colleagues noted that only the southern populations were protected in a conservation area.

The species is in CITES Appendix II, like the whole genus since 4 February 1977, with annotation #4 (version adopted at CoP20, in force since 5 March 2026), which exempts seeds and pollen in particular; international trade in live plants requires a permit. In the European Union it is in Annex B of Regulation (EC) No 338/97, whose current annexes are set by Regulation (EU) 2026/1383, in force since 29 June 2026. In Mexico, the NOM-059-SEMARNAT-2010 list of 2010, which predates the split of Dioon vovidesii, classes it as “in danger of extinction” (category P), and the 2019 update keeps that category. Databases checked on 26 October 2026: check the regulations in force before any purchase, and prefer seed-grown plants.

Growing Dioon sonorense

Dioon sonorense is very rare in cultivation and almost nothing has been published about its needs; the only advice comes from nurseries, and part of it may concern plants of the old concept, which included Dioon vovidesii. The specialist European nursery Cycadales.eu, which sells rooted offsets, presents it as a small species of very dry conditions, with bluish foliage when young that greens with age, easy to grow “as long as it is given heat and water in summer”, with preferably slow-release fertiliser, provided it is protected from cold; it adds that it grows very well on limy soil, is sensitive to excess water in cold weather and is a plant of warm habitats. The Florida nursery Jones Landscaping Nursery (Moore Haven) describes plants with narrow, bluish leaflets that are, in cultivation, “relatively fast-growing and undemanding given adequate drainage”.

The Californian nursery Jungle Music, in its guide to the best cycads for coastal Southern California, mentions two forms, one of them silver-blue, tolerance of sun along the coast and a trunk of two to three feet (60 to 90 cm) on a very old plant, and calls it very rare. The rarity of the species in cultivation and its recent separation from Dioon vovidesii make it essential to check the provenance of plants sold under this name. The soil recipes and watering schedules that circulate for this species rest on no source of its own.

Outdoor and in-ground cultivation

A 1998 photograph shows a plant growing in a private garden in Los Angeles (Virtual Cycad Encyclopedia), and several Californian, Arizonan and Texan gardens declare the species (see below), but no outdoor result has been described in a dated source. No observation of Dioon sonorense has been published for the Jardin zoologique tropical at La Londe-les-Maures, in the Var.

Container cultivation

The plants offered in Europe are small rooted offsets in pots. In climates with frost, a pot with a very free-draining mix, kept frost-free and dry in winter, is the prudent option; Cycadales.eu stresses the risk of excess water in cold weather.

In cultivation in the UK, US and Australia

  • United Kingdom: no British garden declares the species to BGCI PlantSearch.
  • United States: the Montgomery Botanical Center (Coral Gables, Florida) lists five accessions in its online catalogue, three of them from 2004; the Fairchild Tropical Botanic Garden (Miami) listed it in its online catalogue in October 2026. BGCI PlantSearch also lists the Arizona-Sonora Desert Museum (Tucson), Lotusland, the Huntington, the Los Angeles County Arboretum, San Diego Botanic Garden, the San Diego Zoo, Moore Farms Botanical Garden (South Carolina) and the John Fairey Garden (Texas). A BGCI listing is a declaration by the garden, not proof that the plant is alive today, and accessions made before 2018 may include plants now referable to Dioon vovidesii.
  • Australia: no Australian garden declares the species to BGCI.

Propagation

Dioon sonorense is propagated from seed, and Cycadales.eu also sells rooted offsets, which shows that the species can produce basal suckers. Like all cycads, it has separate male and female plants: a male and a female are needed for fertile seed. The 1984 authors germinated seed collected in Sonora at Naples; they note that its seedlings are smaller than those of the populations further south, with very glaucous, narrow leaflets. As seeds are exempt from CITES, they are the simplest way to obtain plants of known origin. No publication deals specifically with germination.

Pests and diseases

No disease or pest specific to the species is documented. Cycadales.eu reports its sensitivity to excess water in cold weather: as in other Dioon, rot of the roots and trunk in cold, waterlogged soil is the main risk in cultivation.

Cold hardiness

How much cold Dioon sonorense can take is not known. The species is often presented as one of the hardiest or most drought-tolerant Dioon, because it was long the northernmost; but no minimum temperature has been published for its localities, no cold trial is documented, and since 2018 the northern populations, with colder winters, belong to Dioon vovidesii. The values that circulate (−3 to −6 °C, or 27 to 21 °F, and USDA zones 9b to 11) are estimates without a source.

Mediterranean and European experience

No cultivation report is documented in France, whether on the Mediterranean coast, the Atlantic coast or inland. In Italy, the seedlings grown at Naples in 1984 are the only documented trace of cultivation.

United Kingdom, United States and Australia

  • Nursery ratings: the European nursery Cycadales.eu gives it as zone 9b, with damage possible below 0 °C (32 °F); the Californian nursery Jungle Music writes that it is “probably cold hardy into the upper teens F”, i.e. about −8 to −7 °C (17 to 19 °F). Both are commercial assessments, without place, date or plant age, and may concern plants of the old concept.
  • No dated report from a British, American or Australian grower has been found.

Toxicity and precautions

No toxicity study has been published for Dioon sonorense, but it should be treated as toxic, like all cycads. Cycads contain toxic glycosides (cycasin, macrozamin), from which methylazoxymethanol is derived, a toxin affecting the liver and the nervous system; the seeds are the most dangerous part. Bonta and colleagues (2019) also report, in western Mexico, hallucinogenic effects locally attributed to the cycads called “peyote”, including Dioon sonorense.

Wear gloves to handle the seeds, and keep plants and seeds out of reach of children and animals. If any part has been eaten, contact a poison centre, a doctor or a vet without delay.

Traditional uses

According to the 2020 revision, the species is used as an ornamental, its trunk is used to make alcohol (sotol and bacanora), and it is used in traditional medicine against insect stings, snakebites and joint pain. Bonta and colleagues (2019) mention the use of the mucilage for joint complaints. These uses were recorded before or during the split of Dioon vovidesii, and the sources do not always say which populations are concerned. Never try to eat or use any part of the plant.

Frequently asked questions

Is Dioon sonorense the hardiest Dioon?

Nothing supports that claim. It was long the northernmost Dioon, hence its reputation, but no cold measurement has been published and the northern populations, with colder winters, became Dioon vovidesii in 2018. Nurseries rate it between zone 9b and the upper teens °F, without data.

Is it still the northernmost Dioon?

No. Since 2018 that title belongs to Dioon vovidesii, which reaches the Sierra de Mazatán, near the 29th parallel. Dioon sonorense in the current sense grows only in southern Sonora and northern Sinaloa, south of about the 28th parallel.

How can it be told from Dioon tomasellii?

By the leaflets and the young leaves. Dioon sonorense has narrow leaflets (less than 7 mm), glaucous, often spiny and spaced, and pale green young leaves; Dioon tomasellii has broader, dark green, drooping, crowded leaflets and young leaves covered in golden-brown down.

Does it tolerate drought?

Probably well, but this has not been measured in cultivation. It grows in seasonally dry forest and scrub on rocky slopes, with a long dry season. The 250 to 500 mm of rain often quoted referred to the old concept of the species, mainly central Sonora.

Can it be grown outdoors in the UK?

Not as far as is known. No trial has been published in Britain or elsewhere in northern Europe; grow it in a pot with a very free-draining mix and keep it frost-free and dry in winter.

Is it toxic?

Yes, it should be treated as toxic, like all cycads, especially its seeds. Keep them out of reach of children and animals, and call a poison centre if any part has been eaten.

Is it endangered?

Yes. It is listed as Endangered by the IUCN and as in danger of extinction in Mexico. Alcohol making from the trunks and habitat loss have destroyed whole populations.

Further reading on Succulentes

Sources

Reference sites

Bibliography

  • De Luca, P., Sabato, S. & Vázquez Torres, M. (1984). Dion tomasellii (Zamiaceae), a new species with two varieties from western Mexico. Brittonia 36(3): 223–227. doi:10.2307/2806511 [Original description of var. sonorense; copy from The Cycad Society.]
  • McVaugh, R. & Pérez de la Rosa, J.A. (1992). Zamiaceae. In: Flora Novo-Galiciana 17: 112. [Combination Dioon edule var. sonorense; cited from The World List of Cycads.]
  • Chemnick, J., Gregory, T.J. & Salas-Morales, S. (1997, issued 1998). A revision of Dioon tomasellii (Zamiaceae) from western México, a range extension of D. merolae, and clarification of D. purpusii. Phytologia 83(1): 1–6. [Raising to species rank, differences from Dioon tomasellii, habitat of the species in its old sense; full text (Internet Archive).]
  • Moretti, A., Caputo, P., Cozzolino, S., Gaudio, L., Siniscalco Gigliano, G. & Stevenson, D.W. (1993). A phylogenetic analysis of Dioon (Zamiaceae). American Journal of Botany 80: 204–214. [Chloroplast DNA analysis; cited from Chemnick et al. (1997).]
  • Gutiérrez-Ortega, J.S., Kajita, T. & Molina-Freaner, F.E. (2014). Conservation genetics of an endangered cycad, Dioon sonorense (Zamiaceae): implications from variation of chloroplast DNA. Botanical Sciences 92(3): 441–451. doi:10.17129/botsci.112 [Two evolutionary units, haplotype shared with Dioon tomasellii; abstract.]
  • Gutiérrez-Ortega, J.S., Jiménez-Cedillo, K., Pérez-Farrera, M.A., Martínez, J.F., Molina-Freaner, F., Watano, Y. & Kajita, T. (2018a). Species definition of Dioon sonorense (Zamiaceae, Cycadales), and description of D. vovidesii, a new cycad species from northwestern Mexico. Phytotaxa 369(2): 107–114. doi:10.11646/phytotaxa.369.2.4 [Redefinition of the species; abstract.]
  • Gutiérrez-Ortega, J.S., Jiménez-Cedillo, K., Pérez-Farrera, M.A., Vovides, A.P., Martínez, J.F., Molina-Freaner, F. et al. (2018b). Considering evolutionary processes in cycad conservation: identification of evolutionarily significant units within Dioon sonorense (Zamiaceae) in northwestern Mexico. Conservation Genetics 19(5): 1069–1081. doi:10.1007/s10592-018-1079-2 [Northern and southern evolutionary units; abstract.]
  • Gutiérrez-Ortega, J.S., Molina-Freaner, F., Martínez, J.F. et al. (2021). Speciation along a latitudinal gradient: the origin of the Neotropical cycad sister pair Dioon sonorense–D. vovidesii (Zamiaceae). Ecology and Evolution 11(11): 6962–6976. doi:10.1002/ece3.7545 [Boundary, habitats, climate, divergence; open-access article.]
  • Bonta, M., Pulido-Silva, M.T., Diego-Vargas, T., Vite-Reyes, A., Vovides, A.P. & Cibrián-Jaramillo, A. (2019). Ethnobotany of Mexican and northern Central American cycads (Zamiaceae). Journal of Ethnobiology and Ethnomedicine 15: 4. doi:10.1186/s13002-018-0282-z [Sotol, mucilage, “peyote” cycads; open-access article.]
  • Hernández-Tapia, J.E., Jiménez-Ramírez, J. & Vovides, A.P. (2020). Taxonomic revision of the genus Dioon (Zamiaceae). Phytotaxa 442(4): 267–290. doi:10.11646/phytotaxa.442.4.2 [Description, range, uses, common names; treatment (Plazi).]
  • Dorsey, B.L., Gregory, T.J., Sass, C. & Specht, C.D. (2018). Pleistocene diversification in an ancient lineage: a role for glacial cycles in the evolutionary history of Dioon Lindl. (Zamiaceae). American Journal of Botany 105(9): 1512–1530. doi:10.1002/ajb2.1149 [Clades of the genus; cited from the 2020 revision.]
  • Bösenberg, J.D. (2022). Dioon sonorense. The IUCN Red List of Threatened Species 2022: e.T42130A69831933. doi:10.2305/IUCN.UK.2022-1.RLTS.T42130A69831933.en [Endangered; reference checked on Crossref.]
  • Haynes, J.L. (2022). Etymological compendium of cycad names. Phytotaxa 550(1): 1–31. doi:10.11646/phytotaxa.550.1.1 [Etymology, as given by The World List of Cycads.]
  • Calonje, M., Stevenson, D.W. & Osborne, R. (2026). The World List of Cycads, version 2026.08.15. Montgomery Botanical Center, Coral Gables. doi:10.5281/zenodo.21958524 [Reference checklist.]