The 9-1-1 complex protects ssDNA gaps in BRCA2-deficient cancer.

Single-stranded DNA (ssDNA) gaps are a hallmark of BRCA-deficient cells, yet the mechanisms that safeguard these lesions remain unclear. Through a genome-wide CRISPR screen, we identified the RAD9A-HUS1-RAD1 (9-1-1) complex as essential for the survival of BRCA2-deficient cells through an ATR-independent mechanism. Loss of 9-1-1 in this context leads to the accumulation of PRIMPOL-dependent gaps that fail to undergo post-replicative repair, resulting in pathological expansion and increased DNA damage. This instability is driven by excessive EXO1-mediated degradation, as EXO1 depletion rescues the phenotype. We further demonstrate that the 9-1-1 complex is required for POLζ-dependent gap filling. We propose a model in which ssDNA gaps, when extended beyond a critical length, become inaccessible to TLS-mediated repair and are fully reliant on homologous recombination. These findings establish the 9-1-1 complex as key regulator of ssDNA gap stability and a promising therapeutic target in BRCA2-deficient cancers.