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Variable and constant parts of the immunoglobulin light chain gene of a mouse myeloma cell are 1250 nontranslated bases apart.

A DNA fragment carrying an immunoglobulin gene coding for both variable (V) and constant (C) regions of a mouse lambda light chain was enriched about 15-fold from a total endonuclease EcoRI digest of a plasmacytoma (HOPC 2020) DNA by preparative agarose gel electrophoresis. The DNA fraction was used for cloning of a lambda chain gene in the phage lambdagtWES vector. After screening [Benton, W. D. & Davis, R. W. (1977) Science 196, 180-182] of about 70,000 plaques, each arising from an independent transfection event, we isolated one clone (Ig 303) that contained both a Vlambda and a Clambda DNA sequence. Electron microscopy of R-loops formed between the cloned DNA and purified lambda chain mRNA (HOPC 2020) revealed that the Vlambda and Clambda DNA sequences are separated by a 1250-base DNA fragment.