Thalassemia is a heterogeneous group of hereditary anemias that arise due to insufficient synthesis or absence of one or more polypeptide chains of globin in the hemoglobin molecule. The defect can affect any of the four polypeptide chains of normal hemoglobins – α, β, γ, or δ – determining the type of thalassemic syndrome. The most common types are α- and β-thalassemias. In Bulgaria, the most frequently encountered type is β-thalassemia in its heterozygous form.