VAV1 and skin neoplasm: We hypothesized that Vav proteins could be also involved in the TPA-dependent promotion phase of skin tumors, an idea consistent with our prior observations indicating that the effect of the double Vav2;Vav3 gene deficiency in the reduction of tumor burden was significantly more conspicuous in DMBA/TPA- than in DMBA/DMBA-treated mice (Figure 1; compare panels B and H).