STAT5 is also involved in transformation by the v-Src oncogene. (A) The tyrosine phosphorylation status of STAT3 and STAT5 was analyzed in NIH 3T3 cells and two derivatives that were transformed by the v-Src oncogene. STAT3 and STAT5 are both constitutively phosphorylated on tyrosine in v-Src–transformed, but not parental NIH-3T3 cells. (B) NIH 3T3 cells were transfected with v-Src expression plasmid and increasing concentrations of DN-STAT3 and DN-STAT5 expression vectors or wild-type STAT5 as a control. Two weeks after transfection, foci were scored and represented as percent compared with v-Src alone. Both DN STAT3 and DN STAT5 partially block transformation of NIH 3T3 by v-Src, although DN STAT3 is much more potent. Wild-type STAT5 could overcome the effect of STAT5δ750, but not STAT3β. (C) NIH-3T3 cells were transfected with the STAT3-dependent SIE-CAT reporter plasmid and increasing amounts of STAT3β or STAT5δ750. Only STAT3β is able to block v-Src–induced SIE-CAT activity.