ESR1 and breast neoplasm: The PR status is specifically important for distinguishing between breast tumor subgroups that might benefit differently from adjuvant anti-estrogenic therapies: the ER+/PR- subgroup was recognized as less responsive to endocrine therapy (particularly tamoxifen) than ER+/PR+ subgroup [5, 7–9], and it has been shown that the strength of nuclear ER and PR staining is related to the likelihood of benefit from these drugs [10].