EM127 (compound 11c) is a SMYD3 covalent inhibitor with high selectivity, high affinity ( K D =13 μM) and site-specificity. EM127 effectively inhibits ERK1/2 phosphorylation and reduces transcriptional regulation of SMYD3 target genes. EM127 effectively and prolongedly impairs methyltransferase activity. EM127 can be used in cancer research, particularly in SMYD3 positive tumours.