Given the task definition and input, reply with output. In this task, you are given a sentence. You are expected to recognize the name of gene or protein. Although there might be several correct answers, you need to write one of them.

A detailed histopathologic review of 50 primary breast cancer tumors analyzed for estrogen receptor ( ER ) level was carried out and a variety of morphologic features correlated with ER results to identify any factors that will improve the management and prognosis for breast cancer .
estrogen receptor