AQP4 encodes a member of the aquaporin family of intrinsic membrane proteins that function as water-selective channels in the plasma membranes of many cells. Aquaporin 4 is the predominant aquaporin found in brain and has an important role in brain water homeostasis. Alternatively spliced transcript variants encoding different isoforms have been described for AQP4. A recent study provided evidence for translational readthrough in AQP4 and expression of an additional C-terminally extended isoform via the use of an alternative in-frame translation termination codon.