We also found that ER+/HER2− breast cancer PDXs with low levels of phospho-CHK2 are more likely to be derived from patients with premenopausal breast cancer (Fig. 8G; median age for phospho-CHK2 low PDXs is 38.5 versus 57 in pCHK2 high lines, P = 0.009) in accordance with data from genetically engineered mice presented in Figs. 3 and 4 and the mutational analysis of germline CHEK2 variants in patient data presented above. This evidence concerns the gene ESR1 and breast cancer.