This peptide is a fragment of the human aquaporin-2 (AQP2) phosphorylated at Ser261. Protein phosphorylation plays a key role in vasopressin signaling in renal-collecting duct. Phosphorylation at several AQP2 residues including Ser256 and Ser261, is altered in response to vasopressin. It is possible that both sites are involved in vasopressin-dependent AQP2 trafficking.