We discovered that the E3 ubiquitin ligase RAD18, which monoubiquitinates PCNA at lysine 164 (38), and the E2-conjugating enzyme UBC13, which promotes further K63-linked polyubiquitination of PCNA (39–41), promote fork recovery in BRCA1- but not BRCA2-deficient cancer cells. Here, BRCA1 is linked to cancer.