A thalassemia severity score (TSS) was developed from 890 homozygous β-thalassemia patients of the Mediterranean basin using these different genetic modifiers including sex, β-thalassemia mutations, α-thalassemia, the Xmn1-HBG2, rs1427407 and rs1018957 in BCL11A and rs9399137 in HBS1L-MYB17. The gene discussed is HBS1L; the disease is thalassemia.