EIF2AK4 and viral infectious disease: These four kinases respond to different stimuli: general control nonderepressible 2 (GCN2) is typically activated by amino acid depletion, PKR-like endoplasmic reticulum kinase (PERK) is activated by endoplasmic reticulum stress, protein kinase R (PKR) is activated by viral infection, and heme-regulated inhibitor (HRI) is activated by heme deficiency [50-54].