Exon trapping was used to identify portions of genes on human chromosome 21. BLAST search of databases revealed that the trapped sequence hmc02a08 (one of 559 different trapped exons) was identical to a region of GenBank entry U61166 for an SH3 (Src-homology) domain-containing gene SH3D1A (formerly called SH3P17; Sparks et al., 1996). We subsequently mapped this gene to YACs and cosmids within 21q22.1→q22.2 between DNA markers D21S319 and D21S65 using hybridization and PCR amplification.