The omega/lambda 5 surrogate immunoglobulin light chain is expressed on the surface of transitional B lymphocytes in murine bone marrow.

The membrane immunoglobulin heavy chain (micron) plays a feedback role during the pre-B stage of B lymphocyte differentiation. In pre-B cell lines, micron associates with two surrogate light chain proteins. The omega chain is disulfide linked to mu and was predicted to be the product of the lambda 5 gene. The iota chain is noncovalently associated with micron. We demonstrate that the omega protein is indeed the product of the lambda 5 gene and that mu, omega, and iota are coassociated in the same complex. Antibodies against the omega/lambda 5 protein demonstrate the existence of a subpopulation of "transitional" bone marrow B cells that express micron and omega on the cell surface. The majority of these cells also express surface kappa light chains, indicating that in B lymphoid ontogeny the lambda 5 gene is inactivated after the onset of kappa light chain expression.