ESR1 and breast cancer: Next-generation sequencing (NGS) studies, while confirming the relative rarity of RAS/RAF/MEK/ERK gene alterations in breast cancer [5], have often identified driver mutations in these genes (Table 2), for instance of KRAS and more rarely of BRAF, particularly in triple negative breast cancer (TNBC) (estrogen receptor-, progesterone receptor-, and HER2-negative) [68,69,70].