The percentage of radiotherapy non-compliance reported ranged between 27-46% in the analysis of seven retrospective studies, with the exception of one study that reported a compliance rate of 86% [36]. While another study found that treatment with tamoxifen, a hormone therapy drug that blocks estrogen receptors in ER+ cancers after BCS to help prevent cancer reoccurrence [37], was discontinued in 20.3% of 64 MBC patients at a median time of 49-months [35]. Reasons for discontinuation most often included weight gain (22%) and sexual dysfunction (22%) [35]. This evidence concerns the gene ESR1 and sexual dysfunction.