To determine if targeting NEK2 may be selective for cancer versus normal cells, we treated several breast cancer cell lines (MDA-MB-231, SUM159, HCC1143, and MCF7) as well as nontransformed mammary epithelial cells (MCF10A and MCF12A) with increasing doses of a NEK2 inhibitor [NBI-961 (49)] and assessed growth. Here, NEK2 is linked to breast cancer.