The major translational products of each of the eight members of the Src gene family identified to date are membrane-associated tyrosine protein kinases that lack transmembrane and external amino acid sequences, thereby distinguishing this group from the receptor class of tyrosine kinases. Members of this group include c-Src, c-Yes, Fyn, Lck, Lyn, Blk and c-Fgr. The major Src gene encoded protein, c-Src (also designated pp60Src, Src p60 and proto-oncogene tyrosine protein kinase Src) is expressed in a broad range of tissue and cell types, although the highest levels of c-Src are detected in neuronal tissues and platelets. c-Src may play a role in events associated with both neuronal differentiation and maintenance of mature neuronal cell functions.