In undifferentiated healthy cells and in some tumor cells, cIAP1 is expressed in the cell nucleus12, 13, 14, 15 where it interacts with E2F1 protein.12 In this study, we demonstrate that cIAP1 induces an accumulation of E2F1 modified through K63 ubiquitination on the lysine cluster 161/164, which is preceded by an arginine methylation step. Here, E2F1 is linked to neoplasm.