I-138 is an orally active compound structurally related to ML323 . I-138 and ML-323 are potent reversible inhibitors of USP1-UAF1. I-138 displays synergistic binding with ubiquitin and mutual exclusive binding with ML323. I-138 induces the monoubiquitination of FANCD2 and PCNA in MDA-MB-436 cells, increases PCNA and FANCD2 monoubiquitination in HAP-1 USP1 WT cells. I-138 ablates USP1 autocleavage in cells.