AID mRNA expression levels in control tissue, purified GC and naive B cells, MCL tissue, and B-CLL tissue. All MCL and B-CLL lymph node tissue samples were analyzed using standard histology and immunohistochemistry including CD5, CD23, and cyclin D1. Tissue samples contained less than 5% CD3⁺ T cells. Pre-existent germinal centers were not present in the studied MCL and B-CLL tissue samples upon histologic examination. By immunohistochemistry there were no CD10⁺ germinal center cells present. For all MCL cases, t(11;14) was confirmed by fluorescence in situ hybridization. Expression of ZAP-70, a surrogate marker for germ line IGVH status in B-CLL,^8,9  was assessed by immunohistochemistry. Taqman quantitative RT-PCR for AID (Applied Biosystems, Foster City, CA; Assay-on-demand Hs00221068_m1; this primer set detects all known alternative splice variants) and TATA-box binding protein (TBP) as endogenous control was performed on reactive tonsils and lymph nodes (LN), purified GC and naive B cells, and tissue samples from MCL patients and B-CLL patients. By dividing the mean threshold cycle (Ct) from triplicate AID measurements by the mean threshold cycle from triplicate TBP measurements, the mean ΔCt was calculated. The ΔΔCt values were calculated using the mean ΔCt of the 2 naive B-cell samples and the 3 healthy donor peripheral blood mononuclear (PBMNC) cell samples as a calibrator. The expression factor difference and range were calculated by the following formulas: 2^-ΔΔCt (mean factor difference); 2^{-(ΔΔCt-ΔCt SD)} and 2^{-(ΔΔCt+ΔCt SD)} (error bars indicate range factor difference). The factor difference conversion of the ΔΔCt and range are depicted in the graph in a logarithmic fashion. ^* indicates not detectable.