MMP9 and acquired polycythemia vera: In the context of defective cell adhesive properties, several proteins have been described to be involved [25], the common denominator being that the large majority are constituents of neutrophil granules [21]–[24] Thus, plasma levels of neutrophil elastase and of metalloproteinase (MMP-9) have been found to be increased in PMF and PV patients with the highest values being recorded in patients with PMF [25].