In PMF, tight clusters of TGFβ1 positive neoplastic megakaryocytes found immediately adjacent to bone distinguish PMF from other types of MPN (Figure 1), suggesting TGFβ1 signaling to NRP2-expressing osteolineage and profibrotic MSC within the endosteal niche. The gene discussed is TGFB1; the disease is myeloproliferative disorder.