Although the number of NK cells also decreases after chemo-irradiation [63], their numbers may still have remained sufficiently high to eradicate 4T1 breast cancer upon their cytokine-induced activation, analogous to the observation that i.v. injected MHC-mismatched hematopoietic stem cells are rejected by host NK cells even in lethally irradiated mice (the so-called “hybrid resistance model” [66]). This evidence concerns the gene HLA-C and breast cancer.