The BLK gene is a nonreceptor protein tyrosine kinase with a calculated molecular mass of about 58 kD. It has an overall amino acid identity of approximately 87% to the mouse Blk; however, in the unique domain of Src family members there is only 58% homology and an insertion of 6 amino acids in the N-terminal region. The nature of this insertion suggested a functional role in membrane attachment. Mutation screening of each exon by direct sequencing of genomic DNA from KWE patients did not reveal any pathogenic mutation. Because BLK is a member of the SRC family, which is thought to play an important role in the signaling pathways controlling cell proliferation and differentiation, the authors considered the gene to be a good positional candidate for the cancers mapping to this region.