Four fusion proteins—ETV6 (TEL)-JAK2 (t(9;12) (p24;p13), PCM1-JAK2 (t(8;9) (p22;p24), BCR-JAK2 (t(9;22) (p24;q11.2), and RPN1-JAK2 (t(3;9) (q21;p24)—have been described in some MPNs—PMF, chronic eosinophilic leukemia (CEL), atypical chronic myeloid leukemia (aCML), and unclassified MPNs—or in mixed MPN/myelodysplastic syndrome (MPN/MDS)34–37. The gene discussed is RPN1; the disease is atypical chronic myeloid leukemia, BCR-ABL1 negative.