High rates of deletions in the constant region segment of the immunoglobulin mu gene.

Spontaneous deletions at the immunoglobulin heavy-chain locus are frequently found in myelomas, hybridomas and pre-B-cell lines. We have measured the rates for large and small deletions within the constant-region gene segment for mu chain in a pre-B-cell lines. The large deletions, which include the entire first and second exons, occurred at a rate of 1.7 X 10(-5) per cell generation. The small deletions, which span a few base pairs, occurred at a rate of 1.4 X 10(-7) per cell generation. The rate for the reversion of a termination codon in the second exon is even less than that for the small deletions and is 1000 times lower than the reversion rate that had been determined for the variable-region gene segment. Therefore, the variable-region gene segment is likely to be the preferred target for hypermutation.