MRK-740 is a potent, selective and substrate-competitive PRDM9 histone methyltransferase inhibitor with an IC50 of 80?nM. MRK-740 is more selective for PRDM9 than other histone methyltransferases and other non-epigenetic targets. MRK-740 reduces PRDM9-dependent trimethylation of H3K4 (IC50?=?0.8?μM).