A mouse myeloma variant with a defect in light chain synthesis

A spontaneous assembly variant, B 50, has been isolated from the MPC‐11 mouse myeloma cell line. This variant synthesizes fewer light chains per cell than the parent resulting in the production of a slight molar excess of heavy chains. These changes are associated with a delay and change in the pathway of assembly and a delay in secretion. Spontaneous revertants of B 50 have been obtained, all of which synthesize normal amounts of light chains and assemble and secrete the immunoglobulin molecule through the same pathways and with the same kinetics as the parental cells. A comparison of the tryptic‐chymotryptic peptides of the parental, variant and revertant heavy and light chains did not reveal any differences. These studies indicate that variants in mouse myeloma cells can arise with defects in the quantitative expression of the immunoglobulin gene and suggest that the presence of excess light chains facilitates the assembly and secretion of some immunoglobulin molecules.