HDAC9 and ovarian cancer: The inhibition of HDAC enzymes led to a significant reduction in the transcription of HR genes, including BRCA1 and RAD51, and sensitized HR‐proficient ovarian cancer cells to PARP inhibition both in vitro and in vivo.72, 73 Furthermore, combining PARP inhibition with metformin enhanced the anti‐tumour effects of PARP inhibitors in both BRCA‐deficient and BRCA‐proficient ovarian cancer cells.74 DNA methylation plays an essential role in regulating normal biological processes as well as carcinogenesis.