The FN2 of MMP9 was found to have affinity for denatured collagen.20 Using FASTSNP software to predict functional effects of the amino acid change from glutamine to arginine induced by rs2664538, this gene variation seems highly likely to change protein structure and exonic splicing.21 In our study, we aimed to evaluate MMP2 and MMP9 polymorphism and disease associations in patients with PV, ET, and secondary polycythemia (SP). Here, MMP9 is linked to secondary polycythemia.