NI-42 (compound 13-d), a structurally orthogonal chemical probe for the BRPFs, is a biased, potent inhibitor of the BRD of the BRPFs (IC50s of BRPF1/2/3=7.9/48/260 nM; Kds of BRPF1/2/3=40/210/940 nM) with excellent selectivity over nonclass IV BRD proteins[1].