CHEK2 and breast cancer: These data are supported by previous studies showing that (i) mammary tumors growing in CHEK2*1100delC mice crossed with the MMTV-Ron kinase mammary tumor–susceptible mouse line are highly metastatic (42); (ii) there is increased incidence of germline CHEK2 mutations in metastatic breast cancer relative to primary disease (54); and (iii) that CHEK2 mutation carriers have worse recurrence-free breast cancer survival outcomes (55).