ML323 is a potent, reversible, inhibitor of USP1-UAF1 with IC₅₀ of 76 nM in a Ub-Rho assay and 174 nM and 820 nM in orthogonal gel-based assays using K63-linked diubiquitin (di-Ub) and monoubiquitinated PCNA (Ub-PCNA) as substrates, respectively.