Phospholipase C-gamma 1 (PLC-gamma 1) is a multiple-domain protein and plays an important role in epidermal growth factor (EGF)-induced cell mitogenesis, but the underlying mechanism is unclear. We have previously demonstrated that PLC-gamma 1 is required for EGF-induced mitogenesis of squamous cell carcinoma (SCC) cells, but the mitogenic function of PLC-gamma 1 is independent of its lipase activity. Earlier studies suggest that the Src homology 3 (SH3) domain of PLC-gamma 1 possesses mitogenic activity. In the present study, we sought to determine the role of the SH3 domain of PLC-gamma 1 in EGF-induced SCC cell mitogenesis. We examined the effect of overexpression of PLC-gamma 1, a catalytically active PLC-gamma 1 mutant lacking the SH3 domain or a catalytically inactive PLC-gamma 1 mutant lacking the X domain on EGF-induced SCC4 (tongue squamous cell carcinoma) cell mitogenesis. We found that overexpression of PLC-gamma 1 enhanced EGF-induced SCC4 cell mitogenesis. This enhancement was abolished by deletion of the SH3 domain but not by deletion of the X catalytic domain. These data suggest that the SH3 domain, but not the catalytic domain, is required for PLC-gamma 1 to mediate EGF-induced SCC4 cell mitogenesis. (c) 2010 Elsevier Inc. All rights reserved.