Human ABCG4 shows sequence homology to the Drosophila white gene, the product of which must dimerise to become functionally active. ABCG4 is closely related to ABCG1 with 74% identity and is thus thought to play a role in the efflux of excess cholesterol (Engel et al. 2001). Northern Blot analysis shows that ABCG4 is expressed specifically in brain and the eye (Oldfield et al. 2002).