B-CLL can up-regulate AID on appropriate stimulation in vitro. RT-PCR results from purified B cells from 8 B-CLL samples, 5 mutated and 3 unmutated (italicized). RNA was extracted from purified B-CLL cases before (day 0, first panel from the top) and after 3.5 days of stimulation with either PMA and ionomycin (third panel) or CD32-transfected L cells with anti-CD40 mAb and IL-4 (fourth panel). Cells cultured in medium alone (second panel) or with only CD32-transfected L cells (not shown) remained RT-PCR negative throughout the culture period. AID amplification was performed using the primers 63F and 415R for 33 cycles. β-actin was positive and qualitatively similar in intensity for all tested samples (data not shown).