Molecular characterization of a mutant hybridoma secreting truncated IgM.

We have identified the mutation in an IgM hybridoma mutant line which produces truncated mu protein. DNA sequencing has revealed that two base pairs are deleted in the Cmu3 domain, resulting in an early termination of protein synthesis by forming an in-frame termination codon (UGA). The mutant mu chain is incorporated into monomeric IgM which is secreted efficiently.