Mammalian multidrug and toxic compound extrusion 1 (MATE1) are polyspecific H+-coupled exporters of organic cations (OCs) and responsible for excretion of metabolic waste products and xenobiotics. Here, we report a novel variant of mouse MATE1, mMATE1b, that has a long carboxyl terminal hydrophobic tail homologous to other MATE1 transporter proteins. Mouse MATE1b mediates tetraethylammonium (TEA) uptake with properties similar to that of mMATE1 and is localized in renal brush border membranes. Thus, mMATE1b is a functional variant of mMATE1 and seems to be the true counterpart to other MATE1 transporters.