They defined four PV subgroups based on the quantitative relationship between JAK2 V617F and 9p aUPD: 42% of patients with heterozygous JAK2 V617F and no detectable 9p aUPD (subgroup I); 45% of patients with homozygous JAK2 V617F and an allelic fraction directly proportional to the level of 9p aUPD (subgroup II); 10% with 9p aUPD at approximately twice the level of heterozygous JAK2 V617F allelic burden (subgroup III) and 3% with trisomy 9p and two copies of JAK2 V617F allele (subgroup IV), which likely suggest different pathways leading to PV phenotype. This evidence concerns the gene JAK2 and acquired polycythemia vera.