ESR1 and breast cancer: This study will consider the presence of the aforementioned genes in (a) the differential gene expression derived from an Affymetrix, and (b) the presence of such genes in breast cancer patients, taking into account (i) the estrogen receptor α (ESR1) gene expression, (ii) gene expression of tumor and normal tissues, (iii) the estrogen receptor (ER) status, (iv) the protein levels, (v) immune cell infiltration analysis in breast cancer subtypes, and (vi) overall survival.