Nested PCR amplification of switch fragments. Serially diluted aliquots of DNA from PBMCs of patient no. 2 (left) or control (right), cultured with medium or with IL-13 plus anti-CD40 MoAb, were amplified by nested primer PCR. The quantities of serially diluted template DNA used in the first round of PCR are noted above the gel; in the second round of PCR 10% of the original PCR reaction mixture was used as DNA template. Final PCR products were analyzed by agarose gel electrophoresis. In the control lane (C), primers specific for the human IL-1β promoter yielded the expected 1.4-kb fragment. Lane MW represents the molecular-weight markers.