EIF2AK4 and infection: Furthermore, given that extracellular levels of TRP must be below 3 μM in vitro in order to activate the GCN2 stress pathway in T cells (9), and the fact that we detected levels of TRP in the tissues well above this concentration throughout infection (100 μM for spleen, 20 μM for brain), it seems unlikely that TRP depletion is a major contributor to T cell dysfunction during HIV/SIV infection.