AQP4 and neuromyelitis optica: At least 40 different mutations in the AQP2 gene (resulting in protein product variants) have been linked to the onset of autosomal nephrogenic diabetes insipidus, where loss of responsiveness to vasopressin disrupts the kidneys’ ability to concentrate urine, resulting in an excessive excretory volume.42 Although site-specific mutations in the AQP4 molecule have been associated with reduced binding of potentially pathogenic IgG (in conjunction with the neurodegenerative disease neuromyelitis optica),43 clinically relevant immune responses to mutant AQPs remain unknown.