20.5 kb of sequence including codons 96-2389, comprising exons 3-5 and part of exon 6, were deleted upon the insertion of a neomycin resistance cassette. Transcripts containing exon 5 sequences were not found in analysis of mutants, and both heavy and light chain polypeptides were absent from immunoblots of mutants. Also absent were any truncated amino-terminal proteins encoded by exons 1 or 2. (J:97423)