Method and performance of PCR used to detect B cells with Cμ deletion. (A) Schematic representation of the Cμ-Cδ switch region of the C_H gene. Recombination between donor and acceptor switch regions (S-S recombination) is the basis for Ig class switch. Because no authentic Sδ region exists, IgD expression depends on recombination between Sμ and an intronic Cμ-Cδ sequence (σδ), thereby deleting Cμ and a variable part of the Sμ region. Amplification of Sμ-σδ recombination was obtained by nested PCR with primer positions as indicated. (B) DNA samples from 12 consecutive tissue samples analyzed as shown above. Variable break points give rise to dissimilar size of the PCR products, thereby revealing several clones depending on the frequency of their local occurrence. MLN indicates mesenteric lymph node; IgA-d, IgA-deficient samples.