The study explores the mechanism behind the synthetic lethality of PARP inhibitors with homologous recombination (HR)-deficient cancers. It is found that PARP1, TIMELESS, and TIPIN protect the replisome from transcription-replication conflicts (TRCs) in early S phase. The synthetic lethality of PARP inhibitors with HR deficiency is due to the inability of HR-deficient cells to repair DNA damage caused by TRCs, rather than by trapped PARPs. Inhibiting transcription elongation in early S phase renders HR-deficient cells resistant to PARP inhibitors, and depletion of PARP1 is synthetic lethal with HR deficiency. These findings suggest that inhibiting PARP1 enzymatic activity may be sufficient for treatment efficacy in HR-deficient settings. The study also highlights the importance of understanding how PARP inhibitors target HR-deficient cells, including the role of PARP1, to guide their clinical development.The study explores the mechanism behind the synthetic lethality of PARP inhibitors with homologous recombination (HR)-deficient cancers. It is found that PARP1, TIMELESS, and TIPIN protect the replisome from transcription-replication conflicts (TRCs) in early S phase. The synthetic lethality of PARP inhibitors with HR deficiency is due to the inability of HR-deficient cells to repair DNA damage caused by TRCs, rather than by trapped PARPs. Inhibiting transcription elongation in early S phase renders HR-deficient cells resistant to PARP inhibitors, and depletion of PARP1 is synthetic lethal with HR deficiency. These findings suggest that inhibiting PARP1 enzymatic activity may be sufficient for treatment efficacy in HR-deficient settings. The study also highlights the importance of understanding how PARP inhibitors target HR-deficient cells, including the role of PARP1, to guide their clinical development.