Two newly developed techniques have greatly enlarged our knowledge of the basic genetic defects of the beta-thalassemias: 1) Restriction enzyme digestion of cellular DNA followed by analysis of the cleaved fragments. 2) Cloning of beta-globin genes from patients with beta-thalassemia and subsequent determination of the nucleotide sequence of these genes. Some of the results are presented. Only rarely a deletion of the beta-globin gene was found. In most cases, a mutation within the beta-globin gene was discovered as basis for the beta-thalassemia syndrome. These and other findings are a starting point for planning a therapy of beta-thalassemia by means of gene technology: correction of the defect within the abnormal beta-globin gene, replacement of the beta-globin gene, stimulation of gamma-globin synthesis.