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In addition, some of these subunits have more than one isoform encoded by different genes and with different tissue and subcellular expression patterns. Only one form has been identified for the A subunit of the human V-ATPase. This ubiquitous subunit has a nucleotide-binding site and is thought to be the catalytic subunit of the V-ATPase. Subunit B is present in two isoforms: the B1 isoform, originally called the kidney isoform of the 56-kDa subunit, and the B2 isoform, originally called the brain isoform of the 56-kDa subunit. Subunits C, G, and E are part of the peripheral stalk that connects the V1 and V0 domains of the V-ATPase. Two human isoforms are known for the C subunit. C1 is ubiquitously expressed, whereas C2 has so far been detected only in kidney and lung. Three G subunit isoforms have been described: Gl, G2, and G3. G1 is ubiquitously expressed, G2 is brain specific, and G3 has been localized in the kidney. Two E subunit isoforms were identified: the sperm-specific E1 isoform and the ubiquitously expressed E2 isoform (originally called the 31-kDa subunit, or the E subunit). Subunit a is the largest subunit of the V-ATPase and is part of the V0 domain of the pump.