In the classification of the brachydactylies, the analysis by Bell (1951) proved highly useful. The type A brachydactylies of Bell have the shortening confined mainly to the middle phalanges. In the brachydactyly A1 type (BDA1), the middle phalanges of all the digits are rudimentary or fused with the terminal phalanges. The proximal phalanges of the thumbs and big toes are short. Genetic Heterogeneity of Brachydactyly Type A1 BDA1B (607004) has been mapped to chromosome 5. BDA1C (615072) is caused by mutation in the GDF5 gene (601146) on chromosome 20q11. BDA1D (616849) is caused by mutation in the BMPR1B gene (603248) on chromosome 4q22. [from OMIM]