Fig. 3

Model of deletion, fusion, and repeat expansion to form the ultralong DH region in cluster 2. Homologous sequences in clusters could misalign where IGHD5 and 6 pair with IGHD3 and 7 during replication processes. IGHD3 and its 3′ intergenic sequence is deleted (transparent grey rectangle). The short nucleotide repeats found in IGHD6 and 7 could cause mispairing and fusion, creating a fused DH-DH region of IGHD6 and IGHD7. A gene conversion or duplication event of IGHD6 would be required under this scenario. Continued repeat expansion produces IGHD8–2 homologs (red arrow). “pCluster” denotes a hypothetical precursor cluster in evolution