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.

Viral protein U ( Vpu ) is a protein encoded by human immunodeficiency virus type 1 ( HIV - 1 ) that promotes the degradation of the virus receptor , CD4 , and enhances the release of virus particles from cells .
Viral protein U