Members of this family adopt an immunoglobulin-like beta-sandwich, with seven strands in 2 beta sheets, in a Greek key topology. They are predominantly found in the extracellular portion of CD4 proteins, where they enable interaction with major histocompatibility complex class II antigens [1].

1: Crystal structure of domains 3 and 4 of rat CD4: relation to the NH2-terminal domains. Brady RL, Dodson EJ, Dodson GG, Lange G, Davis SJ, Williams AF, Barclay AN; Science. 1993;260:979-983. PMID:8493535