Type B1 brachydactyly (BDB1) is the most severe type of human brachydactyly, and shows high penetrance and variable expressivity. Hypoplastic or absent distal phalanges and nails of digits 2 through 5 in the hands and feet are cardinal phenotypic features of BDB1. The middle phalanges of digits 2 through 5 are usually short and may form a bony fusion with the corresponding hypoplastic distal phalanges. The deformed thumbs are often flat, broad, or bifid. A rarer feature of BDB1 is cutaneous syndactyly affecting both fingers and toes (summary by Lv et al., 2009). [from
OMIM]