ESR1 and obesity due to melanocortin 4 receptor deficiency: Weight gain and obesity is an important factor to consider in breast cancer and has been correlated with poor breast cancer outcomes.36 Increased adiposity increases levels of estradiol, which can then in turn stimulate cell growth through multiple pathways, including those mediated through the estrogen receptor itself, and growth factor-dependent pathways.37, , –40 Among the three groups studied, we found no significant difference in weight gain or loss despite differing steroid doses.