PPM1D and cancer: To study the effect of Wip1 knockdown on phosphorylated UBE4B in cancer cell line, MCF-7 stable clones expressing Wip1-shRNA (sh-Wip1) or ctl-shRNA were exposed to 10 J/m2 UV and harvested at different time points, and analyzed by immunoblotting with antibodies against phospho-UBE4B S669, UBE4B, Wip1, p53, and phospho-p53 S15.