Immunoglobulin heavy chain allelic exclusion is maintained in the absence of Btk and PLCγ2. Genomic DNA was extracted from bone marrow CD19⁺ IgL⁻ cells of various mice and was serially diluted and subjected to PCR using primers to amplify D_H-to-J_H and V_HJ558-to-D_HJ_H gene arrangements. The PCR products were analyzed by Southern blotting using a DNA fragment spanning J_H3 to J_H4 as probe. The GADPH gene was amplified as control for the amount of genomic DNA used. Data shown are representative of 2 independent experiments.