Eg5 role in BC is also related to the molecular and histological classification of BC according to which three different types of BC can be recognized: BC positive for estrogen receptors (ER) and progesterone receptors (PR) (ER+, PR+), BC positive for ER, PR, and for human epidermal receptor 2 (HER2+), BC positive only for HER2 and triple negative BC (TNBC, negative for ER, PR, and HER2 receptors) [83]. Here, ESR1 is linked to breast cancer.