Simultaneous expression of mouse immunoglobulins M and D is determined by the same homolog of chromosome 12.

A hamster-mouse hybrid cell line expressiong both murine IgM and murine IgD on the membrane was shown to have only one copy of mouse chromosome 12. This chromosome is known to carry the structural genes for the immunoglobulin heavy chains. Cloning of populations selected for loss of mouse membrane IgM yielded cells that had also lost expression of membrane IgD, but not the expression of hamster immunoglobulin heavy chain. Karyotype analysis of these subclones demonstrated the concurrent loss of the chromosome 12 present in the parental hybrid. Absence of this chromosome was confirmed by use of the isozyme acid phosphatase 1. The results of the genetic analysis prove that the coexpression of mu and delta immunoglobulin heavy chains is not due to long-lived immunoglobulin mRNA nor to the transcription of genes on homologous chromosomes. We conclude that the genetic information for IgM and IgD expressed by a single cell lies on the same chromosome.