EIF2AK4 and viral infectious disease: Four stress-responsive eIF2α kinases are conserved in mammals and activated by specific stressors: PKR-like endoplasmic reticulum kinase (PERK) by ER stress, general control nonderepressible-2 (GCN2) by amino acid starvation, heme-regulated inhibitor of translation (HRI) by heme deficiency, and dsRNA-activated protein kinase (PKR) by viral infection [107].