XRCC1 depletion enhanced PARPi-induced cell cycle arrest. (A and B) Representative cell cycle profiles of C4-2B (A) and 22RV1 (B) vector control cells and XRCC1-KO C-1 following 48 h treatment with 0.5 μM rucaparib using PI DNA staining-based flow cytometry. (C and D) Percent distribution of cells in G1, S, and G2/M phase of the cell cycle. (E and F) Immunoblots (left) and quantification (right) show the expression of cell cycle arrest-related proteins in C4-2B (E) and 22RV1 (F) vector control cells, XRCC1-KO C-1, and XRCC1-KO C-1 + XRCC1 after 48 h treatment with 0.5 μM rucaparib. Actin was used as a loading control. The level of statistical significance among the groups was computed using two-way ANOVA with Tukey’s multiple comparisons test. The level of statistical significance is indicated as follows: *P < 0.05, **P < 0.01, and ***P < 0.001.