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The presence of G2 in the epididymis was not investigated here because this subunit has been previously shown to be specific for the brain and absent from the kidney. In addition to their localization in the apical plasma membrane and subapical vesicles, a faint cytosolic staining was also observed for A, C1, C2, G1, and G3, similarly to the previously reported localization of other subunits of the V1 domain, including subunit E2, in the soluble fraction of clear cells. Many subunits of the V0 sector, including a1, a4, d1, and d2, were also expressed in narrow and clear cells. a1 and a4 were located in their apical pole but showed distinct subcellular distribution. a4 showed the brightest staining and the closest colocalization with subunit E2 in subapical vesicles and apical microvilli. Expression of subunit a4 has also recently been described in the mouse epididymis. Thus a4 appears to be the predominant isoform in narrow and clear cells. The ubiquitously expressed a1 isoform was present in subapical vesicles but was absent from apical microvilli. Both d1 and d2 were found in the apical pole of narrow and clear cells, where they might participate in endosomal acidification and/or proton secretion.