RAD51 and cancer: Genomic alterations, gene mutations or functional loss of proteins involved in DDR mechanisms, such as ATM, ATR, CHEK1, CHEK2, DSS1, RAD51, MRE11A/NBS1, Fanconi anemia complementation group (FANC family of genes), EMSY, PALB2, XRCC2, XRCC3, or PTEN, could represent predictive markers in cancer patients to tailor a personalized treatment with PARP inhibitors [129–141].