PARP1 and small cell lung carcinoma: Moreover, PARP inhibition itself induces HR deficiency by reducing the expression of the E2F1 target genes involved in DNA replication and cell cycle regulation (e.g., PCNA, MCM7, and CCNA2) and HR factors such as BRCA1/2 and RAD51, as shown in prostate and small cell lung cancer (Byers et al. 2012; Schiewer et al. 2018).