UNC926 is a small molecule inhibitor of the methyl-lysine reader proteins L3MBTL1 (IC50: 3.9 µM) and L3MBTL3. It selectively disrupts the interaction between the L3MBTL1 3xMBT domain and the histone mark H4K20me1 without affecting 53BP1 binding, making it a useful chemical probe for epigenetic and cancer research.