NSC 663284 (DA-3003-1) is a potent, cell-permeable, and irreversible Cdc25 dual specificity phosphatase inhibitor, has an IC50 for Cdc25B2 of 0.21 μM. NSC 663284 exhibits mixed competitive kinetics against Cdc25A, Cdc25B(2), and Cdc25C with Ki values of 29, 95, and 89 nM, respectively. NSC 663284 inhibits NSD2 (IC50 of 170 nM) through a direct interaction with the catalytic SET domain (Kd of 370 nM).