JAK2 and polycythemia: Interestingly, the 46/1 haplotype, which was most strongly associated with PV in the MPN patient cohort, was not significant for PV in the less well-defined UKBB-PV cohort (Fig. 3a), in which the majority of individuals were JAK2V617F negative, suggesting that this locus is not a risk factor for developing high red blood cell indices independently of mutant JAK2, such as in secondary or apparent polycythemia.