Mutations in adult hemoglobin alpha genes in humans lead to blood disorders commonly known as α-thalassemia. In search of a mouse model for this disease, mutagenesis screens have identified several deletions that resemble these phenotypes. The Hba b2(th) deletion, in particular, replicates the characteristics of alpha-thalassemia minor in heterozygous mice but presents a homozygous embryonic lethal phenotype. Previous analyses...