H eme-regulated eIF2α kinase (HRI) controls protein synthesis by phosphorylating the α-subunit of eukaryotic translational initiation factor 2 (eIF2α). In heme deficiency, HRI is essential for translational regulation of α- and β-globins and for the survival of erythroid progenitors. HRI is also activated by a number of cytoplasmic stresses other than heme deficiency, including oxidative stress and heat shock. However, to date, HRI has not been implicated in the pathogenesis of any known human disease or mouse phenotype. Here we report the essential role of HRI in 2 mouse models of human rbc disorders, namely erythropoietic protoporphyria (EPP) and β-thalassemia. In both cases, lack of HRI adversely modifies the phenotype: HRI deficiency exacerbates EPP and renders β-thalassemia embryonically lethal. This study establishes the protective function of HRI in inherited rbc diseases in mice and suggests that HRI may be a significant modifier of many rbc disorders in humans. Our findings also demonstrate that translational regulation could play a critical role in the clinical manifestation of rbc diseases.