Studies of paired samples of the primary tumor and locally/regionally recurrent or distant metastases suggest that tumor receptor status may be discordant in a significant proportion of patients: 18% to 54% for ER, 36% to 54% for PR, and 3% to 22% for HER2 in both retrospective series [6,10-14] and small prospective series [15-17]. Here, ESR1 is linked to neoplasm.