The six subfamilies of SLC22 transporters. Evolutionary analysis indicates that SLC22 transporters are highly conserved and found in fly, worm, sea urchin, and other organisms. The SLC22 family consists of two major clades: OAT (organic anion transporter) and OCT (organic cation transporter). Each of these clades can be further divided into three subclades, designated as OAT, OAT-like, OAT-related, OCT, OCTN (organic cation/carnitine transporter), and OCT/OCTN-related.