One study found that nine of 12 NMO patients and seven of eight LETM patients had severe salivary gland inflammation (a hall mark of Sjögren syndrome).[49] AQP4 is expressed in low levels in salivary glands whereas AQP5, which shares 50% protein sequence with AQP4, is expressed at high levels and plays a major role in salivary gland secretion. Here, AQP4 is linked to neuromyelitis optica.