JAK2 and acquired polycythemia vera: In contrast, JAK2 exon 12 mutations are specific for PV, with 3% positivity (Figure 1A), and a variety of mutations have been identified at JAK2 exon 12 (Supplementary Table S1, according to the Catalogue Of Somatic Mutations In Cancer (COSMIC) database, as of June 2023) [20,21].