Genetic studies have identified 3 major quantitative trait loci (Xmn1-HBG2, HBS1L-MYB intergenic region on chromosome 6q23, and BCL11A on chromosome 2p16) that account for 20% to 50% of the expected variation in HbF levels in patients with SCA, β-thalassemia, and healthy adults. This evidence concerns the gene HBS1L and autosomal dominant cerebellar ataxia.