Characterization of a seven-copy IGHV3-23 haplotype. (A) Structural variant giving a seven-copy haplotype of IGHV3-23 in HD2. Illustration shows the order of the seven homologous segments. (B) Synteny plot showing sequence identity and structural variation between the seven consecutive IGHV3-23-containing segments. Similarity between segments is represented by lines colored by the percentage identity of matched bases in 250 bp bins. (C) V gene usage in IgG1 transcripts derived from each haplotype in HD2, a “canonical” two-copy and the novel seven-copy IGHV3-23 haplotype. (D) Frequency of allele usage in IgM transcripts containing IGHV3-23 from the seven-copy haplotype.