Gelatinase B/MMP-9 involvement in malignant tumour progression has now moved on from original concepts of an almost exclusive role in matrix degradation, associated with tumour invasion, to include roles in almost all aspects of tumour biology, ranging from initiation and early progression, to angiogenesis, dissemination, invasion and motility, formation of the cancer stem cell niche, regulation of tumour immunological surveillance, metastatic site preparation and promotion of metastatic growth. The gene discussed is MMP9; the disease is cancer.