Schematic representation of the primary structures of mammalian UT-A isoforms. UT-A1 comprises four hydrophobic regions. UT-A2, UT-A3 and UT-A4 each comprise two hydrophobic regions, which are identical to regions in UT-A1, as indicated by matching coloured boxes. UT-A5 and UT-A6 are identical to UT-A3 except for a unique N-terminus in UT-A5 and a unique C-terminus in UT-A6. Coloured boxes represent hydrophobic regions, grey lines represent regions with common DNA coding sequences, the dashed black line connects regions that are continuous with one another, asterisks represent regions that have unique coding sequences. Abbreviation: UT, urea transporter. Permission obtained from Wiley © Smith, C. P. Mammalian urea transporters. Exp. Physiol. 94, 180–185 (2009).