Expression of AID mRNA, somatic hypermutation, and class switch recombination of the

IgH gene in MALT lymphoma and immunocytoma. cDNA pools were synthesized from total RNA of MALT lymphomas (n = 7) and immunocytoma (n = 7). AID (nt 200-520), blimp-1 (421-824), and β-actin cDNA (nt 391-660) were amplified by PCR and hybridized with a radiolabeled internal oligonucleotide. Recombined V(D)J segments of the IgH transcripts were amplified using a set of degenerate oligonucleotides as primers and subsequently entirely sequenced. Point mutations of less than 3% in comparison to the published sequences are defined as somatic hypermutation negative (−), point mutations of more than 3% but less than 6% are defined as somatic hypermutation positive (+), and point mutations of more than 6% are defined as strong positive for somatic hypermutation (++). Immunoglobulin heavy-chain expression was determined by standard immunohistochemistry, and the constant region of the IgH locus is indicated. n.d. denotes nondetectable IgH gene expression.