Analysis of mutations introduced into the chromosomal immunoglobulin mu gene

We have introduced a pSV2neo-derived vector that contains a 2-base-pair (bp) deletion in its immunoglobulin mu gene constant region into hybridoma cells bearing a single copy of the wild-type chromosomal immunoglobulin mu gene. Homologous recombination between the transferred mutant C mu region and the wild-type chromosomal C mu region is expected to introduce the 2-bp deletion into the chromosomal mu gene, generating recombinant cells synthesizing noncytolytic IgM. Analysis of the DNA in independent noncytolytic transformants indicates that in one case the mu gene has the structure expected for correct homologous recombination. Unexpectedly, the remaining transformants, bear chromosomal mu gene deletions.