These studies are directed toward determining the molecular lesion affecting beta-globin production in patients with homozygous beta-thalassemia. Earlier work in this laboratory and in others has established that ineffective removal of intervening sequence RNA from beta globin mRNA precursors is a frequent cause of beta globin mRNA deficiency in patients with severe beta thalassemia. Our efforts during the past year have focused on attempts to define abnormal RNA precursors in bone marrow cells in patients with beta thalassemia and to purify the globin genes from such patients by molecular cloning. Two types of abnormal RNA precursors have been demonstrated; one involves an abnormal splice within the first smaller intervening sequence and the second apparently involves a failure of ligation of an RNA molecule improperly spliced within the second larger intervening sequence. A new cloning strategy has been adapted in our laboratory which should facilitate rapid isolation of thalassemic globin genes.