ESR1 and breast cancer: Interestingly, we observed that their different ability to affect MCF7 (ER+, PR+, HER2−), SKBR3 (ER−, PR+, HER2+) and MDA-MB231 (ER−, PR−, HER2−) BC cell growth was not due to a lack of expression of 5-HT2 receptors but eventually attributable to their different molecular features, which also give them a different degree of aggressiveness.