You will be given a definition of a task first, then some input of the task.
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.

Since the rise in E2 - receptor display paralleled a large increase in the proliferative activity of the cells as well as a change in their maturation status the question was raised as to whether the E2 - receptor should be considered as a physiological marker of growth rate or of cellular differentiation .
Output:
E2 - receptor