The amino acid sequence has been completed for the constant region of the Fd fragments of heavy chains from rabbit IgG (immunoglobulin G) of allotype Aa1 and Aa3. The amino acid sequence given by Fruchter et al. [(1970) Biochem. J. 116, 249-259] for the constant region of the Fd fragment from Aa1 IgG was extended and in in part corrected to give a continuous sequence of 140 residues. No allotype-related sequence variation was found in the constant section of the Fd fragment. This evidence confirms the view that the differences in sequence between the variable regions of Aa1 and Aa3 IgG [Mole et al., (1971) Biochem. J. 124, 301-318] are responsible for the allotypic specificities.
