JAK2 and thrombocythemia 3: In addition, germline variants in the JAK2 gene are causative for thrombocythemia 3 (OMIM ID: 614521) (Mead et al., 2012; Amberger et al., 2019); and in several cases, germline JAK2 variants, also in combination with other variants, were responsible for the activation of JAK2/STAT signaling in hereditary erythrocytosis or PV patients (Kapralova et al., 2014, 2016; Milosevic Feenstra et al., 2016; Wu et al., 2018).