Mice that are homozygous for the targeted mutation are viable and fertile, but are anemic. Reduced levels of gene product (mRNA) are detected by Northern blot analysis of homozygous embryos (due to compensatory expression of the _2-globin gene). Homozygous crosses yield smaller litters. Homozygotes exhibit increased platelet number, reduced hemoglobin concentration and mean corpuscular volume. Heterozygotes are viable, fertile, normal in size and exhibit an intermediary phenotype. This mutant mouse strain may be useful in studies of erythropoiesis, _-Thalassemia and hemoglobin defects. The Jackson Laboratory 2024, used with permission.