EIF2AK4 and viral infectious disease: Three other eIF2α kinases (EIF2AKs) can phosphorylate Ser51 of eIF2α to initiate the ISR: PKR (Double-stranded RNA-dependent protein kinase) is activated by viral infection, HRI (Heme-regulated eIF2α kinase) by Haem depletion and GCN2 (General control non-depressible protein 2) by amino acid starvation.