I-138 is an orally active, potent reversible inhibitor of USP1-UAF1 with IC50 0f 4.1 nM. I-138 is structurally related to ML323 and induces the monoubiquitination of FANCD2 and PCNA in MDA-MB-436 cells and increases PCNA and FANCD2 monoubiquitination in HAP-1 USP1 WT cells.