In addition to large-scale studies by The Cancer Genome Atlas (TCGA) and the International Cancer Genome Consortium (ICGC), which have characterised the molecular genetic status of primary breast cancer in detail [34,35], the sequencing of 617 breast cancer samples identified nine genes (TP53, ESR1, GATA3, KMT2C, NCOR1, AKT1, NF1, RIC8A and RB1) that were more frequently mutated in metastatic breast cancer compared to early breast cancer [36]. This evidence concerns the gene ESR1 and breast cancer.