ATP-dependent transporter of the ATP-binding cassette (ABC) family that may be involved in the cellular efflux of sterols, in particular cholesterol and desmosterol (a cholesterol precursor), to high-density lipoprotein (HDL) (By similarity). May play an important role in the removal of amyloid-beta peptides from brain,in a process that can be antagonized by desmosterol. However it is unclear whether ABCG4 can directly transport amyloid-beta peptides or whether peptide export may be facilitated due to changes in the membrane lipid environment (By similarity). Induces apoptosis in various cells (By similarity). ATP + cholesterol(in) + H2O = ADP + cholesterol(out) + H(+) + phosphate ATP + desmosterol(in) + H2O = ADP + desmosterol(out) + H(+) + phosphate Half-transporter that form a functional transporter via homo- or heterodimerization. Homodimer. Heterodimers with ABCG1. Expressed specifically in the brain and the eye (PubMed:12183068). Expressed in both neonatal and adult oligodendrocytes (PubMed:21972082). Whether ABCG4 is an LXR target gene, is still under debate. Studies performed in monocytes, and in one astrocyte cell line indicated that ABCG4 expression could be up-regulated by oxysterols and other LXR ligands (By similarity). However, subsequent observations in a number of different cell types (primary mouse cells, oligodendrocytes and neuron-like cell lines) have not confirmed this observation (By similarity) (PubMed:21972082). Belongs to the ABC transporter superfamily. ABCG family. Eye pigment precursor importer (TC 3.A.1.204) subfamily.