JAK2 and Splenomegaly: Osm deficiency also protected against splenomegaly and myelofibrosis induced by JAK2 p.V617F: Osm-/- mice displayed no signs of fibrosis after more than 200 days, whereas mild to intermediate fibrosis developed in 4/7 Jak2 p.V617F+Osm+/+ mice (Fig. 6e, f).