PARP inhibitors induce sensitivity in HR-deficient cancers through transcription-replication conflicts (TRCs). This study shows that PARP1, along with TIMELESS and TIPIN, prevents TRCs by protecting the replisome during early S phase. The synthetic lethality of PARP inhibitors with HR deficiency is due to the inability to repair DNA damage caused by TRCs, not PARP trapping. Inhibiting transcription elongation in early S phase makes HR-deficient cells resistant to PARP inhibitors, and PARP1 depletion is synthetic lethal with HR deficiency. HR-deficient cells are highly sensitive to PARP inhibitors because they cannot repair DNA damage from TRCs. PARP1 and PARP2 bind to DNA damage sites, triggering a conformational switch that traps PARPs on DNA. However, PARP inhibitors may enhance, neutralize, or reduce PARP retention on DNA depending on their effect on the DNA binding domain. The mechanism of PARP inhibitor-induced lethality in HR-deficient cells is attributed to TRCs, not PARP trapping. PARP1 interacts with TIMELESS to prevent TRCs. Depletion of TIMELESS or TIPIN leads to TRCs, DNA damage, and synthetic lethality with HR deficiency. PARP1 and TIMELESS act through the same pathway to prevent TRCs. Inhibiting PARP1 enzymatic activity is sufficient to induce TRCs, as depleted PARPs cannot be trapped. PARP inhibitors induce TRCs in early S phase, leading to DNA damage that requires HR for repair. The synthetic lethality of PARP inhibitors with HR deficiency is due to TRCs, not PARP trapping. PARP1 depletion mimics PARP inhibitor effects, and TRC-induced DNA damage is repaired in HR-proficient cells. PARP trapping reduces the selectivity of PARP inhibitors for HR-deficient cells. PARP inhibitors with higher enzymatic activity are more effective against HR-deficient cells. Depleting PARP1 increases the sensitivity of HR-deficient cells to PARP inhibitors. PARP1-selective inhibitors, such as saruparib, may be more effective in clinical settings. The study highlights the role of TRCs in PARP inhibitor-induced synthetic lethality in HR-deficient cells. Understanding this mechanism could improve the clinical development of PARP inhibitors, particularly in terms of isoform specificity and trapping potential. The findings suggest that reducing PARP trapping may decrease toxicity without compromising efficacy.PARP inhibitors induce sensitivity in HR-deficient cancers through transcription-replication conflicts (TRCs). This study shows that PARP1, along with TIMELESS and TIPIN, prevents TRCs by protecting the replisome during early S phase. The synthetic lethality of PARP inhibitors with HR deficiency is due to the inability to repair DNA damage caused by TRCs, not PARP trapping. Inhibiting transcription elongation in early S phase makes HR-deficient cells resistant to PARP inhibitors, and PARP1 depletion is synthetic lethal with HR deficiency. HR-deficient cells are highly sensitive to PARP inhibitors because they cannot repair DNA damage from TRCs. PARP1 and PARP2 bind to DNA damage sites, triggering a conformational switch that traps PARPs on DNA. However, PARP inhibitors may enhance, neutralize, or reduce PARP retention on DNA depending on their effect on the DNA binding domain. The mechanism of PARP inhibitor-induced lethality in HR-deficient cells is attributed to TRCs, not PARP trapping. PARP1 interacts with TIMELESS to prevent TRCs. Depletion of TIMELESS or TIPIN leads to TRCs, DNA damage, and synthetic lethality with HR deficiency. PARP1 and TIMELESS act through the same pathway to prevent TRCs. Inhibiting PARP1 enzymatic activity is sufficient to induce TRCs, as depleted PARPs cannot be trapped. PARP inhibitors induce TRCs in early S phase, leading to DNA damage that requires HR for repair. The synthetic lethality of PARP inhibitors with HR deficiency is due to TRCs, not PARP trapping. PARP1 depletion mimics PARP inhibitor effects, and TRC-induced DNA damage is repaired in HR-proficient cells. PARP trapping reduces the selectivity of PARP inhibitors for HR-deficient cells. PARP inhibitors with higher enzymatic activity are more effective against HR-deficient cells. Depleting PARP1 increases the sensitivity of HR-deficient cells to PARP inhibitors. PARP1-selective inhibitors, such as saruparib, may be more effective in clinical settings. The study highlights the role of TRCs in PARP inhibitor-induced synthetic lethality in HR-deficient cells. Understanding this mechanism could improve the clinical development of PARP inhibitors, particularly in terms of isoform specificity and trapping potential. The findings suggest that reducing PARP trapping may decrease toxicity without compromising efficacy.