ESR1 and breast cancer: Notably, normal-adjacent tissues in ER-positive subtype had higher numbers of circRNAs than tumor samples in TCGA, and the number of circRNAs in normal-adjacent samples of the ER+ subtype was inversely correlated to the risk-of-relapse proliferation (ROR-P) score for proliferating genes, suggesting that the circRNA frequency might be a marker for cell proliferation in BC.