Immunoblots showing Cks1, Skp2, and p27Kip1 proteins in breast cancer cell lines. (a) Levels of the three proteins Cks1 (cyclin kinase subunit 1), Skp2 (S-phase kinase protein 2), and p27Kip1 were measured in MDA-MB-231 (an estrogen-independent cell line) and MCF-7 (an estrogen-dependent cell line). Skp1 levels were determined to confirm equal protein loading. (b)Effect of estrogen modulation on Cks1, Skp2, and p27Kip1 expression in T47D cells (an estrogen-dependent breast cancer cell line) treated with estradiol (0.5 μM) or tamoxifen (10 μM). P27, p27Kip1.