MMP9 and neoplasm: However, several studies provided contradictory results, that is, a protective role of host MMPs in carcinogenesis, tumour growth or metastases by the experiments using MMP-3 KO (McCawley et al, 2004), MMP-7 KO (Acuff et al, 2006a), MMP-9 KO (Cornelius et al, 1998; Hamano et al, 2003) and MMP-12 KO mice (Acuff et al, 2006b).