ESR1 and obesity due to melanocortin 4 receptor deficiency: These data suggest that in the setting of obesity, upregulation of ER levels in BC cells by SFA stimulated Mφ may enable their proliferative response even to extremely low levels of estrogen, thus providing a molecular/cellular explanation for the observation that obese patients are more likely to experience ER+BC recurrence after treatment with aromatase inhibitors [23] and generally contributing to the worse response to hormonal therapy in BC [13,19,79].