A variable region subclass of heavy chains common to immunoglobulins G, A, and M and characterized by an unblocked amino-terminal residue.

Heavy chains of IgG, IgA, and IgM classes of human immunoglobulins were compared by N-terminal residue determination and partial amino acid sequence analysis. A third subclass of the variable region of heavy chains was observed; an unblocked glutamic acid as the N-terminal residue is characteristic of this subclass. Our results indicated that the heavy chain variable region subclasses are not class specific, and that a given heavy chain variable region may be found in association with constant regions for mu-chains, alpha-chains, or gamma-chains of various subclasses.