Similar genomic organization of Pcdh and UGT1 gene clusters. Shown are comparisons of the genomic organization of mouse Pcdh (A), and mouse (B), rat (C), and human (D) UGT1 clusters. Each cluster has multiple, highly similar, tandem variable exons followed by one set of constant exons. They are indicated by vertical colored bars: (mauve) mouse Pcdhα variable exons; (yellow) C-type Pcdh variable exons; (green) phenol-type UGT1 variable exons; (orange) bilirubin-type UGT1 variable exons; (blue) pseudogenes or relics (present in both Pcdh and UGT1 clusters); (turquoise) non-UGT1 genes in the UGT1 cluster; (pink) constant exons (present in both Pcdh and UGT1 clusters). The approximate length of each cluster is shown below the corresponding panels. The pseudogenes are represented by a letter “p” following the gene symbol, whereas the relic sequences are represented by a letter “r.” (Pcdh) Protocadherin; (UGT) UDP glucuronosyltransferase; (Hsp40L) heat-shock protein 40kd like gene.