Mice that are homozygous for this targeted mutation have an embryonic lethal phenotype. No gene product (mRNA) is detected by Northern blot analysis of homozygous embryos. _-globin mRNA expression is also reduced in homozygous embryos. Homozygous embryos (12 day pc) are anemic, pale, small in size, with peripheral blood anisocytosis, poikilocytosis, and polychromasia and reduced hemoglobin concentration and mean corpuscular volume. Heterozygotes are viable, fertile, and normal in size. Heterozygotes exhibit increased platelet number, and an intermediary phenotype of reduced hemoglobin concentration and mean corpuscular volume. This mutant mouse strain may be useful in studies of erythropoiesis, _-Thalassemia and hemoglobin defects. The Jackson Laboratory 2024, used with permission.