Our data demonstrate that the interference with OPN production (through ERK1/2 inhibition) or activity (through anti-OPN antibodies) constrains the development of BM and spleen fibrosis but does not affect the extramedullary hematopoiesis -and therefore splenomegaly- in a model of constitutive activation of Mpl signaling. This evidence concerns the gene SPP1 and Splenomegaly.