Thalassemias are hereditary anemias caused by mutations that disturb the normal 1:1 balance of alpha- and beta-globin chains that form hemoglobin. We have disrupted the major adult beta-globin gene (b1) in mouse embryonic stem cells by using homologous recombination to insert selectable sequences into the gene. Mice homozygous for this insertional disruption of the b1 gene (Hbbth-2/Hbbth-2) are severely anemic and die perinatally. In contrast, approximately 60% of mice homozygous for deletion of the same gene (Hbbth-1/Hbbth-1) survive to adulthood and are much less anemic [Skow, L. C., Burkhart, B. A., Johnson, F. M., Popp, R. A., Goldberg, S. Z., Anderson, W. F., Barnett, L. B. & Lewis, S. E. (1983) Cell 34, 1043-1052]. These different phenotypes have implications for the control of beta-globin gene expression.