Instructions: 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.
Input:  IL - 2 expression in CD4 + T cells is primarily controlled at the level of transcription , but mechanisms restricting IL - 2 production in CD8 + T cells have not been elucidated .
Output:
IL - 2