Structure of the AID gene, wild-type and splice variant mRNAs, and translated proteins. (A) Genomic structure of AID. The AID gene is composed of 5 exons and spans approximately 11 kilobase (kb). Open boxes represent 5′ and 3′ untranslated regions. Closed boxes represent coding regions, with the lengths of the exons indicated below. (B) Wild-type AID mRNA and splice variants. The splice variants result from either retention of intron 4 in the 939-bp splice variant or omission of exon 4 in the 530-bp variant. (C) Alignment of the wild-type AID amino acid sequence and the putative protein products of the splice variants. Only nonidentical residues are indicated. The wild-type mRNA encodes a 198–amino acid protein that contains a cytidine deaminase domain, indicated by an open box. The splice variants contain premature stop codons, indicated by asterisks. If translated, the 939-bp and 530-bp splice variants would yield truncated proteins of 187 and 145 amino acids, respectively.