Structure of various HCD proteins compared with that of the corresponding normal chain. Shown are the HCD proteins for which the complete sequence is known. V indicates variable region; D, diversity segment; J, joining region; H, hinge region; and CH1, CH2, CH3, and CH4, constant regions of heavy chains. Light turquoise indicates sequences derived from variable region; turquoise, sequences derived from CH domains or hinge region; hashed boxes, sequences derived from D or J segments; purple, unusual amino acid sequences; and dark turquoise, amino acids of the membrane-bound form of HC. Adapted from Buxbaum and Alexander.7 Note that α1SEC exists only in the membrane form.