PARP1 and cancer: Notably, the expression of CCDC6, MBD1, TDG, LIG4, MDM4, and POLB, which are hardly expressed in MDA-MB-231, were significantly increased as the substrate specificity of the 5'-nucleotidases became narrower, whereas PCNA, FEN1, CDC20, CDC25AB, PLK1, and PARP1, which are upregulated in human cancer, were markedly decreased (Fig. 6 and Supp.