Comparison of mouse immunoglobulin y2a and y2b chain genes suggests that exons can be exchanged between genes in a multigenic family ( Jacob, December 19, 1980 ABSTRACT A 23-kilobase EcoRI DNA fragment coding for the BALB/c immunoglobulin y2a chain was cloned from mouse embryo DNA in the cosmid pJC74, and a nucleotide sequence of 1904 bases was determined, for the entire constant region (CHI, CH2, and CH3), the three intervening sequences (IVS 1, IVS 2, and