ATF1 and melanoma: [2], [33]. To test if ATM is a direct target of CREB/ATF1 we transfected human MeWo melanoma cells with siRNA directed against CREB and ATF1 either individually or in combination. While ATF1 knockdown and CREB knockdown significantly reduced ATM mRNA levels, a double knockdown caused an almost 10-fold reduction in ATM mRNA levels (Figs. 5B and C). ATM protein levels were also significantly reduced in CREB/ATF1 knockdown cells (Fig. 5D).