CHEK1 and breast cancer: To confirm this observation, two structurally unrelated Chk1 kinase inhibitors PF-477736 [26] and AZD7762 [27] potently inhibited the proliferation of TNBC breast cancer cell lines compared to two ER-positive cell lines suggesting that the anti-proliferative effects observed were due to Chk1 inhibition and not the inhibition of an off target kinase.