RAD51 and cancer: Decreased levels of HR factors, such as BRCA and RAD51, were observed in cancer cells during hypoxic exposure [98], mainly caused by epigenetic silencing of the BRCA promoter or the transcriptional repression of the RAD51 promoter via an E2F4/p130 complex, a transcription factor induced by hypoxia that occupies the promoter to inhibit access of other transcriptional activators [99], compromising the HR pathway (Figure 4).