MMP9 and neoplasm: The impressive quantity of data concerning gelatinase B/MMP-9 involvement in the different phases of tumor progression, reviewed in this article, highlights particularly important roles for inflammatory leukocyte-derived gelatinase B/MMP-9 in tumour initiation and early progression and a more complex involvement of gelatinase B/MMP-9 from inflammatory, stromal and tumour sources in the continued progression of tumours to metastasis.