CLOCK and breast cancer: Treatment of the ERα-positive breast cancer cell lines, MCF-7 and T47D cells with 1 μM E2 for 24 h resulted in increased expression of CLOCK protein in these cells, but the same treatment given to the ERα-negative cell lines, MDA-MB-231 and MCF10A resulted in no apparent effect on CLOCK expression (Figs. 2A&B).