PCNA and breast cancer: The annotation of the ‘cell cycle’ signature revealed a highly significant overrepresentation of genes involved in: DNA replication and DNA metabolism, e.g. several MCMs, PCNA, RFC2 and RFC4; cell cycle control, e.g. CCND1 and CCNE2, encoding cyclins D1 and E2, respectively, and CDKN1A, the gene encoding p21Waf1/Cip1, which forms a central node in this network; and cancer, including breast cancer.