Alvarez-Larrán et al. investigated the influence of the JAK2 haplotypeGGCC_46/1 during disease follow-up revealing that PV patients homozygous for the haplotype show a progressive and higher increase in the JAK2 V617F allele burden during the disease evolution without cytoreductive therapy, as compared to patients with a negative or heterozygous haplotype [43]. The gene discussed is JAK2; the disease is acquired polycythemia vera.