A truncating allele of the cell cycle checkpoint kinase CHK2 is present in 1% of the population, conferring a moderate increase in breast cancer risk, and inactivation of chk2 enhances mammary tumorigenesis in mice with targeted inactivation of brca1. We used the mouse mammary tumor virus (MMTV) promoter to target expression of a kinase-dead CHK2 allele (D347A). Mammary tumors, of predominantly micropapillary histology, developed in 40% of MMTV-CHK2-D347A transgenic mice with an average latency of 20 months. Tumors metastasized to lung and spleen; tumor-derived cell lines were frequently aneuploid and showed suppression of irradiation-induced p53 function. Primary hematopoietic malignancies were also observed in the spleen, another site of MMTV expression. The increased rate of tumor formation in MMTV-CHK2-D347A mice, compared with the relatively low incidence in chk2-null mice, provides a model to study modifiers of CHK2-dependent transformation.