Despite the presence of dedicated l-lactate/H+ co-transporters of the human monocarboxylate transporter family MCT [16,17], indirect evidence is accumulating that the l-lactic acid permeability of AQP9 may play a role in the astrocyte-to-neuron l-lactate shuttle [18], and AQP9 expression levels may determine the severity of brain tumors [19]; for instance, during tumorigenesis, glioma cells increase AQP9 expression, and it has been suggested that the upregulation of AQP9 enhances the uptake of l-lactate as a nutrient for glioma cells, promoting tumor cell growth [20]. The gene discussed is AQP9; the disease is neoplasm.