The human alpha globin gene cluster located on chromosome 16 spans about 30 kb and includes seven loci: 5'- ζ - pseudo ζ - μ - pseudo α₁ -α₂ -α₁ - θ₃'. The α₂ (HBA2) and α₁ (HBA1) coding sequences are identical. These genes differ slightly over the 5' untranslated regions and the introns, but they differ significantly over the 3' untranslated regions. Two α-chains plus two-β chains constitute HbA, which in normal adult life comprises about 97% of the total hemoglobin; alpha chains combine with delta chains to constitute HbA-2, which with HbF (fetal hemoglobin) makes up the remaining 3% of adult hemoglobin. Alpha thalassemias result from deletions of each of the alpha genes as well as deletions of both HBA2 and HBA1; some nondeletion alpha thalassemias have also been reported.