The rapid cell cycle arrest in response to DNA damage is mediated by degradation of the Cdc25A phosphatase, a proto-oncogene whose mRNA is frequently overexpressed in human tumours. Here, we study the occurrence and mechanisms of Cdc25A deregulation in human breast cancer cell lines. We demonstrate aberrantly elevated Cdc25A protein abundance and phosphatase activity in eight out of 15 cell lines, in some cases resulting in abrogation of the Cdc25A-mediated checkpoint response to ionizing radiation (IR), and this defect correlated with hypersensitivity to IR. Furthermore, we present evidence that deregulation of Cdc25A occurs predominantly on the post-transcriptional level, as overabundant Cdc25A protein was usually not accompanied by adequate mRNA overexpression. Instead, we demonstrate that aberrantly enhanced protein stability is an important mechanism underlying Cdc25A overabundance in a subset of breast cancer cell lines. Given the frequency of this mechanism, we propose that the DNA integrity checkpoint controlling Cdc25A protein stability might be a common target for deregulation in breast cancer.