The tumor suppressive activities of the Kip-family of cyclin-dependent kinase (cdk) inhibitors often go beyond their role directly regulating the cell cycle. In this study, we show that p27 enhances Rad51 accumulation during repair of double-strand DNA breaks. Progression of platelet-derived growth factor (PDGF)-induced oligodendrogliomas was accelerated in mice lacking the cyclin–cdk binding activities of p27 kip1. To understand how p27 deficiency contributes, cell lines were developed from RCAS–PDGF infection of nestin-tv-a brain progenitor cells in culture. p27 deficiency did not affect cell proliferation in early passage cell lines; however, the absence of p27 affected chromosomal stability. In p27-deficient cells, the activation of Atm and Chk2 and the accumulation of γ-H2AX was unaffected when compared with wild-type cells, and the number of phospho-histone H3 staining mitotic cells was decreased, consistent with G2/M checkpoint activation. However, the percentage of Rad51 foci-positive cells was decreased, and the kinase activity that targets the C-terminus of BRCA2, regulating BRCA2/Rad51 interactions, was increased in lysates derived from p27-deficient cells. Increased numbers of chromatid breaks in p27-deficient cells that adapted to the checkpoint were also observed. These findings suggest that Rad51-dependent repair of double-stranded breaks was hindered in p27-deficient cells, leading to chromosomal instability, a hallmark of cancers with poor prognosis.