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) (PubMed:15240127, PubMed:33141061). 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 (PubMed:27228027). ATP + cholesterol(in) + H2O = ADP + cholesterol(out) + H(+) + phosphate ATP + desmosterol(in) + H2O = ADP + desmosterol(out) + H(+) + phosphate Half-transporter that forms a functional transporter via homo- or heterodimerization. Homodimer (PubMed:27228027). Heterodimers with ABCG1 (Probable) (PubMed:27228027). Expressed specifically in the brain and the eye. Protein expression is stabilized by cellular cholesterol status and cholesterol synthesis intermediates desmosterol, lathosterol and lanosterol. 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 (PubMed:11606068, PubMed:33141061). 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:33141061). Belongs to the ABC transporter superfamily. ABCG family. Eye pigment precursor importer (TC 3.A.1.204) subfamily. Database for mutations in ABC proteins