Southern blot analysis of the erythropoietin gene (digested with Bgl II) in six polycythemic Chuvash patients and some of their first-degree relatives. The higher molecular weight Bgl II fragment is polymorphic, with the ∼15 kb wild allele having an 8 kb partner. A mother of the polycythemic patient in the third pedigree (an obligate carrier assuming autosomal recessive inheritance) is homozygous for the poymorphic allele, while her affected son is heterozygous. These data suggest that neither allele is linked to polycythemia.