VHL and Chuvash polycythemia: VHL allelic disorder: While deleterious heterozygous VHL variants can be diagnostic for VHL syndrome, certain variants in the VHL gene are associated only with an increased risk for autosomal recessive familial erythrocytosis type 2 (ECYT 2) (also known as VHL-associated polycythemia, congenital erythrocytosis type 2, or Chuvash polycythemia).