PCNA and breast cancer: Studies showed that vitamin D increases the expression of genes involved in DNA damage repair such as p53, proliferating cell nuclear antigen (PCNA), and breast cancer 1 (BRCA1) in breast cancer cells77 and ataxia-telangiectasia mutated (ATM) and recombinant DNA repair protein (RAD50) in PECs84, and growth arrest and DNA damage-inducible α (GADD45α) in SCC and ovarian cancer cells85,86.