POU5F1 and embryonal carcinoma: Their findings serve to demonstrate several significant points: (i) endogenous OCT4 in human ESCs can be subjected to modification by Ub; (ii) WWP2 enhances the ubiquitination and degradation of OCT4 protein through the 26S proteasome in a dose-dependent manner; and (iii) down-regulation of WWP2 expression significantly elevates the levels of OCT4 protein in undifferentiated human ESCs and embryonal carcinoma cells.101