PCNA and breast cancer: Besides, the idea that the decrease of the DNMT1/PCNA interactions could be at the origin of the tumor transformation associated with the acquisition of the global DNA hypomethylation phenotype in breast cancer is also supported by the fact that several malignant breast cell lines harbor a lower level of DNA methylation and of DNMT1/PCNA interactions than the MCF10A non-tumorigenic breast cell line6.