Since both melatonin and TWIST1 influence the expression of CLOCK-related genes [50,67,68], and inactivation of CLOCK, CRY1, BMAL1, and PER1 has been proposed as a mechanism increasing breast cancer risk in shift workers [69], we next studied the effect of doxorubicin in the expression of CLOCK genes in MCF-7 cells. This evidence concerns the gene PER1 and breast cancer.