SLC38A7 and central nervous system cancer: This value is much lower than Vt values for tumors, that could be found in literature for animal tumor model [20], or derived from published rate constants for human and rat glioma [21, 22], reflecting lower k1/k2 ratio in normal tissues, that in turn could be explained by differences in expression of amino acid transporters, between normal and tumor cells and their affinity to the ligand.