In the structurally similar ABCG5/ABCG8 asymmetric mammalian lipid transporter, an R263Q mutation in the long linker connecting transmembrane helix 1 (TMH-1) of ABCG8 to the nucleotide-binding domain (NBD) has a loss-of-function phenotype resulting in sitosterolemia (Heimer et al. 2002). This evidence concerns the gene ABCG8 and sitosterolemia.