Our investigations of the association of acidic pHe with the metastasis-related activities of mouse B16 melanoma variants, including the induction of matrix metalloproteinase-9 (MMP-9) expression, found that MMP-9 induction correlated with the metastatic activity of B16 variants and the acceleration of tumor invasion through type IV collagen sheets [9, 10]. The gene discussed is MMP9; the disease is neoplasm.