A mutant allele dosage effect on clinical phenotype has been described in PV, whereby higher mutant allele burden correlates with higher severity of disease.55, 56, 57 Retrospective studies have identified that patients homozygous for JAK2V617F mutations are more likely to progress to post-PV MF.56, 57, 58 Rates of fibrotic transformation have been found to be 11.5% versus 1.4%55 and 23% versus 2%59 for homozygous compared with heterozygous JAK2 mutants, respectively. The gene discussed is JAK2; the disease is acquired polycythemia vera.