During the past few years, intensive research has focused on elucidating the function of two polypeptide domains known as SH2 and SH3 (for src homology). These two domains were originally recognized in the non-catalytic regions of non-receptor tyrosine kinases. It was then discovered that a class of cytoplasmic proteins that complex with ligand-stimulated receptor tyrosine kinases contain SH2 and SH3 domains. Among these proteins are the GTPase-activating protein (GAP) of
p21 ras, phospholipase C-y, and the 85 kD subunit of