ESR1 and breast cancer: The clinical significance of these alterations continues to evolve with newly approved targeted therapies and a range of ongoing investigational trials.<h4>Methods</h4>A retrospective study of 2574 breast cancer (BC) and 1110 gynecologic cancer samples that underwent whole exome and whole transcriptome profiling was conducted to assess the distribution of ESR1 and associated co-alterations in local (primary breast or regional lymph node) versus metastatic BC samples and in the major BC subtypes.