It was generally believed that when the repair of SSBs was blocked by PARP1 inhibition, SSBs would be converted into DSBs in S-phase that can only be repaired by HRR, therefore impaired HRR, as in cancer cells carrying BRCA1 or BRCA2 mutations, would render synthetic lethality with PARP1 inhibition [13], [14]. The gene discussed is PARP1; the disease is cancer.