Cks1, a small protein whose expression is strongly associated with aggressive breast tumors, is a component of cyclin-cdk complexes, as well as the SCF Skp2 ubiquitin ligase. In these studies, we explored its roles in estrogen receptor-positive breast tumor cells. When exposed to the antiestrogen ICI 182780, these cells accumulate in G 1 by reducing the expression of Cks1, and increasing the levels of p130/Rb2, a cdk2 inhibitor and SCF Skp2 target. Heregulin β1 or estradiol abrogate antiestrogen effects by increasing Cks1 expression, down-regulating p130/Rb2 and inducing S phase entry. Depletion of Cks1 in these cells by RNA interference concomitantly decreased Skp2 and up-regulated p130/Rb2 and another SCF Skp2 target, p27 Kip1 . Remarkably , however, Cks1-depleted cells not only exhibit slowed G 1 progression, but also accumulate in G 2 -M due to blocked mitotic entry. Notably, we show that cdk1 expression, which is crucial for M phase entry, is drastically diminished by Cks1 depletion, and that restoration of cdk1 reduces G 2 -M accumulation in Cks1-depleted cells. cdk1 reduction in Cks1-depleted cells is a consequence of a marked decrease in its mRNA and not due to alteration in its proteolytic turnover. Both heregulin β1 and estradiol could neither restore cdk1 nor sustain cycling in Cks1-depleted cells, although classical estrogen receptor function remained unaltered. Cks1 depletion also decreased Skp2 in human mammary epithelial cells without altering cell cycle progression. Thus, the indispensability of Cks1 to the breast cancer cell cycle, versus its redundancy in normal cells, suggests that Cks1 abrogation could be an effective interventional strategy in breast cancer. ©2007 American Association for Cancer Research.