To address this question, we treated the human MDA-MB-231 and mouse M6 (derived from a C3(1)/Tag mammary tumor) TNBC cell lines either alone or in combination with gemcitabine, a drug that inhibits ribonucleotide reductase activity and subsequently promotes DNA damage [26], and with UCN-01, a CHK1 inhibitor [27] that prevents cell cycle arrest leading to deficient DNA repair. This evidence concerns the gene CHEK1 and breast cancer.