ABCG5 and ABCG8 transporters and sitosterolemia-associated mutations: Genomic structure (A), putative topology (B), and predicted amino acid sequences of ABCG5 and ABCG8 (C). ABCG5 and ABCG8 are located on chromosome 2p21 between markers D2S177 and D2S119. (A) ABCG5 and ABCG8 are tandemly arrayed in a head-to-head orientation separated by 374 base pairs. ABCG5 and ABCG8 are both encoded by 13 exons and each spans ~28 kb. (B) The mutations detected in patients with sitosterolemia are indicated on a schematic model of ABCG5 (left) and ABCG8 (right). (C) Predicted amino acid sequence of ABCG5 and ABCG8, which are 651 and 673 residues in length, respectively. Alignment of the inferred amino acid sequences indicates 28% sequence identity and 61% sequence similarity between ABCG5 and ABCG8. Both proteins are predicted to contain six transmembrane segments. The putative transmembrane segments of each protein are indicated by blue (ABCG5) or green (ABCG8) cylinders (B) and lines (C). The Walker A and Walker B motifs are highlighted in yellow and lavender, respectively. The ABC signature sequence (C motif) is indicated in purple.