PRDM9 and cancer: Epigenetic drugs (epi-drugs) which are inhibitors of epigenetic modifications and include DNA methyltransferase inhibitors (DNMTi), histone deacetylase inhibitors (HDACi), histone methyltransferase inhibitors (HMTi) and bromodomain and extra-terminal motif protein inhibitors (BETi), have demonstrated clinical success as anti-cancer agents.