“Monoclonal” IgA(κ) and IgM(κ) present in the serum of a patient (SC) were previously shown to have identical κ light chains and great similarity in the variable portions of their heavy chains. Immunofluorescence technique was used in this study on bone marrow smears of SC to identify the cells producing these proteins. IgA and IgM were found to be produced in separate cells. No cells containing both proteins were detected. These results strongly suggest that the clones of the cells producing SC IgA or SC IgM were derived from a single stem cell and support the view that the variable and constant portions of immunoglobulin chains are regulated by two different genes.

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This work was supported in part by Grant AI-8472 from the National Institute of Allergy and Infectious Diseases, Grant AI-00130 (NIH Training Grant) from the National Institute of Allergy and Infectious Diseases and by Grant CA-5834 from the National Cancer Institute.

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