Correlation of expression of AID and its splice variant with identification of variant tumor-derived isotype switch transcripts and circle transcripts in unmutated and mutated V_H gene t(11;14) mantle cell lymphoma. Mutational status of tumor V_H genes is shown as % homology to germ line V_H gene segment (> 98% homology denotes unmutated status [UM] vs mutated [MUT]). Variant tumor isotype transcripts were identified by amplification with CDR3-specific primers together with downstream isotype-specific C_H primer (ND, not determined). Two AID transcripts, wild-type (wt; 656 bp) and variant (loss of exon 4, -ex4; 454 bp), were identified by RT-PCR and amplified bands of predicted size resolved on agarose gels prior to nucleotide sequence verification. AID expression in the lymphoma cell line Ramos served as a positive control. Amplification of V_H genes was used to confirm integrity of RNA and loading. Comparable amounts of cDNA were used for V_H gene and AID amplification. Weakly amplified bands for variant AID transcripts in 2 cases (MUT case 7 and UM case 12) were confirmed as positive by cloning and sequence analysis. In UM case 11, additional bands in AID amplification were nonspecific as shown by sequence data. In 4 of 4 normal PBMNC preparations, AID expression was not detected under the conditions used (data not shown).