Chk2 is the mammalian homologue of yeast Cds1/Rad53. In response to DNA damage, a checkpoint kinase ATM phosphorylates and activates Chk2, which in turn phosphorylates and activates p53. Chk2 also phosphorylates and activates BRCA1, the product of a tumor suppressor gene that is mutated in breast and ovarian cancer.