PARP1 and breast carcinoma: Notably, depletion of HER2 rescued the breast cancer cells from 5a-induced E2F1 downregulation and also abrogated the effect of 5a on the activation of caspase-7, caspase-3 and PARP (Figure 3e), suggesting that HER2 is the predominant target for 5a-induced cell cycle arrest and apoptosis.