A total of eight mammalian PLC isozymes have been described (PLC (beta)1, PLC (beta)2, PLC (beta)3, PLC (beta)4, PLC (gamma)1, PLC (gamma)2, PLC (delta)1 and PLC (delta)2) with molecular weights ranging from 85 to 150 kDa. The (gamma)-type enzymes are unique in that they contain SH2 and SH3 domains. Moreover, the two (gamma)-type enzymes, but not the (beta) and (delta) isozymes, are subject to activation by a number of protein tyrosine kinases which associate with their SH2 domains and induce their activation by phosphoryation. In contrast, activation of PLC (beta)1, PLC (beta)2 and PLC (beta)3 is mediated by the (alpha) subunits of the Gq class of heterotrimeric G proteins and by certain (beta)(gamma) G protein subunits. The regulatory mechanisms for PLC (delta)1 and PLC (delta)2 are as yet not resolved.