ESR1 and breast cancer: Most 17β-oestradiol signalling occurs through two receptors, oestrogen receptor alpha (ERα) and beta (ERβ), which, in addition to being expressed in the breast and hormone receptor positive breast cancer, are distributed throughout regions important in learning, memory and executive function, such as the hypothalamus, amygdala, hippocampus and dorsolateral prefrontal cortex [18,19,20].