Analysis of simple sequence repeat polymorphism. The pedigree of the affected kindred is shown at the top of the figure. DNA for study was available from two deceased individuals (I-1 and I-2) as well as from the living family members. DNA 5′ to the EpoR gene translation initiation site contains a microsatellite repeat polymorphism18,22,23 and this region of DNA was amplified by PCR and the products fractionated by polyacrylamide gel electrophoresis. Most individuals have two differently sized alleles that correspond to the inherited parental copies of the EpoR gene, one from each parent; two individuals (II-1 and II-4) display only one band, consistent with homozygosity for two alleles of the same size. Six alleles were observed and are numbered in order of decreasing size. The two affected individuals in the second generation (II-1 and II-4) share allele 5 inherited from their affected father (I-1) as well as an allele of the same size from their unaffected mother (I-2). The two unaffected individuals in the second generation (II-2 and II-3) must have inherited allele 5 from their mother (I-2). Thus, allele 5 inherited from the father (I-1) cosegregates with the disease phenotype.