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.