To study the effects of a JAK2V617F-bearing MK niche on MPN disease development in vivo, we crossed mice that bear a Cre-inducible human JAK2V617F transgene (termed Flip-Flop, or FF1) (62) with the Pf4-cre mice [which bear a Cre recombinase driven by the MK-specific platelet factor 4 promoter (63)] to express JAK2V617F specifically in MKs (Pf4+FF1+). The gene discussed is PF4; the disease is myeloproliferative neoplasm.