 Two (IL-17RA and IL-17RD) are found in most vertebrates, whereas the other three (IL-17RB, IL-17RC and IL-17RE) have undergone some losses in vertebrates during evolution.Structure of IL-17 receptors is unique in comparison with all other known receptor families.IL-17RA is by far the largest member of the family and has the longest cytoplasmic tail of the family.