Genomic structure of the Abcg5 and Abcg8 genes showing the site and nature of the trac mutation. (A) Abcg5 and Abcg8 are arranged tandemly head to head in opposite transcriptional orientations (arrows) and span ∼ 42 kb on mouse chromosome 17 from position 84.567 Mb to 84.609 Mb (National Center for Biotechnology Information build 36)23 as shown in the scale at the top of the figure. Individual exons of each gene are numbered and represented as filled rectangles and introns are represented as connecting lines. (B) The trac mutation is a G to A base pair substitution in exon 10 of the Abcg5 gene. The mutation changes a UGG tryptophan (Trp) codon in wild-type (+/+) mice to a UGA stop codon in mutant (trac/trac) mice. (C) The normal mouse ABCG5 protein (NP_114090) is composed of 652 amino acids and contains 6 predicted transmembrane domains (gray cylinders). The premature stop codon introduced by the trac mutation at amino acid position 462 is predicted to truncate the protein after the second transmembrane domain and eliminate 190 amino acids containing 4 additional transmembrane domains (region enclosed in black box).
