TAT-MEK1 is an inhibitor of ERK2 , consisting of TAT and MEK1 (N-terminal), TAT (YGRKKRRQRRR) derived from human immunodeficiency ( HIV-1 ) transcriptional trans activator (TAT), is a cell-penetrating peptide. TAT-MEK1 IC 50 in vitro for ERK2 is 29 μM.