Does tumor acidification correlate with cell invasion?  The image indicates that cells experiencing acidic extracellular pH also express markers of cell invasion. The expression of carbonic anhydrase in (red) acidifies extracellular pH same cells express proteinases in (green), increased proteolytic activity helps cell chew the extracellular matrix and invade. This observation demonstrates in vivo evidence that acidic regions within the tumor are sites of active cell invasion.