Analysis of clonal LCLs for expression of immunoglobulin chains by FACS and Western blot immunodetection. (A) Single cell clones were obtained from limiting dilution assays and tested for surface BCR expression with light chain–specific antibodies by FACS. Six LCL clones are shown as representative examples, together with a control bulk population obtained from EBV-infected BCR⁺ B cells (BCR⁺). The numbers indicate the percentile of cells in the respective quadrants. (B) Single-cell clones no. 16, 54, and 59, which all fail to express λ or κ light chains at their cell surfaces, as shown in panel A, also are negative with regard to intracellular expression of the IgG-HC (50 kDa) as revealed by Western blotting. Clones no. 15 and 26 were immunoglobulin HC positive in Western blotting, but only no. 26 expressed a surface BCR, as shown in panel A. Clone no. 12, which does not express a surface BCR, is shown to be IgG-HC negative but IgM-HC (75 kDa) positive in Western blotting. The lane labeled “Ig” is a positive control with a purified commercial human IgG preparation. Another positive control is a pool of BCR⁺/EBV-infected LCLs. All LCLs are EBV infected, since they express the latent viral gene product EBNA2, which also served as a loading control.