CYCS and breast cancer: In a study conducted by Lewis et al., the 72 h treatment with estradiol 1 nM increased the expression of pro-apoptotic proteins (Bax, Bak, Bim, Noxa, Puma, and p53), enhanced the release of cytochrome c and the cleavage of PARP in an ER–positive and estrogen-deprived breast cancer cell line (MCF-7:5C).