Hb G-San Jose [α2β27(A4)Glu→Gly] was detected in four families and three unrelated individuals from Eastern Sicily. Polymorphic restriction sites within theβ-globin gene cluster bearing the mutation were characterized. A complete association ofβ-G-San Jose alleles with Mediterranean haplotype IV was found in the families examined and the same haplotype was also present in the unrelated individuals. These findings support the hypothesis of an unicentric origin of theβ-G-San Jose mutation which may have arisen in Eastern Sicily.