A total of eight mammalian PLC isozymes have been described (PLC beta1, PLC beta2, PLC beta3, PLC beta4, PLC gamma 1, PLC gamma 2, PLC delta1 and PLC delta2) 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 beta1, PLC beta2 and PLC beta3 is mediated by the alpha subunits of the Gq class of heterotrimeric G proteins and by certain betagamma G protein subunits. The regulatory mechanisms for PLC delta1 and PLC delta2 are as yet not resolved.