Mutations at codon R683 of JAK2, have been previously described in B-ALL (especially Down Syndrome associated B-ALL) and represent a mutational "hotspot"; Some variants at this codon (R683) are known to be activating mutations. Cases of B-ALL with JAK2 mutations tend to also show rearrangement/overexpression of CRLF2; these are potentially targetable pathway alterations. Such cases tend to have a BCR/ABL-like transcriptional signature.

JAK2 is a non-receptor tyrosine kinase that mediates signaling via the JAK-STAT pathway and the somatic, activating mutation V617F in the pseudokinase domain of JAK2 has been reported in over 90% of patients with polycythemia vera, 40-70% of essential thrombocytemia and 40-60% of primary myelofibrosis. JAK2 mutations have not been described in papillary thyroid carcinomas. Moreover, this variant has high prevalence in the East Asian population, and thus is predicted be a germline variant. Clinical correlation with other laboratory findings is recommended.