AID expression is limited to a fraction of the B-CLL clone. (A) A representative (CLL 266) limiting dilution experiment is shown. Each gel lane contains the PCR product derived from a well containing the indicated number of cells. The PCR used primers specific for (left panel) the V_H gene of the clone or (right panel) AID. Thirty-two wells were tested at each dilution. The last lane at the highest cell density (^*) did not contain reverse transcriptase. The anticipated size of the V_H band was approximately 360 bp and approximately 310 bp for AID. (B) Representative analysis of the data generated by the limiting dilution experiments. The fraction of AID⁺ aliquots at each dilution was plotted on a log scale versus the total number of cells assayed at each dilution. The interpolated dilution at which 67% of the tested aliquots are positive is approximately equal to the reciprocal frequency of positive cells in the sample.