PADI4 and neoplasm: As far as we know, there are three suitable candidate enzymes for the formation of the 65 kDa MMP-9 from the corresponding 82 kDa isoform: (1) the urokinase system, (2) the peptidyl arginine deiminases (PAD), responsible of the citrullination of MMP-9 [17], and (3) cathepsin K. All are associated with the tumor and metastatic process and (1) and (3) are able to cleave the 82 kDa MMP-9 to give the corresponding isoform of 65 kDa.