SMAD3 and infection: Transfection of these cells with a Smad3 expression vector caused induction of p21WAF1/CIP1 expression (Figure 4G) while, conversely, knock-down of Smad3 by infection with shRNA retroviruses (pRS-Smad3 n°1 and n°2) showed that induction of p21WAF1/CIP1 by activated Notch1 is Smad3-dependent (Figure 4H).