CA9 and neoplasm: The acidic extracellular microenvironment has been previously reported to enhance tumor cell invasion through the activation of proteases such as cathepsins and MMPs.6, 35 Since our findings indicated that CAIX and MMP14 associate and colocalize at functional invadopodia, and that CAIX contributes functionally to collagen degradation at these sites (see Figure 4), we assessed the role of CAIX and MMP14 on invasion through collagen.