Aquaporin 9 has greater sequence similarity with AQP3 and AQP7 and they may be a subfamily. Aquaporin 9 allows passage of a wide variety of noncharged solutes. AQP9 stimulates urea transport and osmotic water permeability; there are contradicting reports about its role in providing glycerol permeability. Aquaporin 9 may also have some role in specialized leukocyte functions such as immunological response and bactericidal activity.

The predicted 295-amino acid AQP9 protein contains the 6 transmembrane membrane regions and intracellular N- and C-termini characteristic of aquaporins. AQP9 shares greater sequence identity with AQP3 and AQP7  than with other members of the family, suggesting that these 3 proteins belong to a subfamily.