A variant of the MPC 11 cell line, M 311, produces a short immunoglobulin heavy chain. When compared with the parental γ2b heavy chain, M 311 was found to have a carboxyl terminal deletion comprising the C H3 domain. The COOH-terminal cyanogen bromide (CNBr) cleavage fragment of M 311 is identical to a corresponding segment of a parental heavy chain CNBr fragment, with the exception of a substitution of asparagine for lysine at the COOH-terminal residue. This observation enabled prediction of both the parental DNA sequence in this region and the genetic mechanism which generated the variant, a frameshift followed by premature termination. This hypothesis is supported by studies of the DNA sequence of the MPC 11 γ2b constant region gene.
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