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:15210959, PubMed:18039927, PubMed:17916878). 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 (PubMed:29042617). 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 forms a functional transporter via homo- or heterodimerization. Homodimer. Heterodimers with ABCG1. Highly expressed in the brain, in particular in neurons, microglia and astrocytes (PubMed:11856881, PubMed:12183068, PubMed:18039927, PubMed:17916878). Expressed on blood brain barrier endothelial cells (PubMed:29042617). Expressed in the spleen (PubMed:11856881). Highly but transiently expressed in enterocytes and hemopoietic cells populating the liver during development, but is absent when animals are fully developed. Highly expressed in the eyes of the developing embryos as early as 12.5 dpc and developing CNS. Abcg4 deficiency does not significantly affect the levels of sterols in the brain except for brain lathosterol levels, which are slightly elevated (PubMed:18039927). Abcg1/Abcg4 double knockout mice display significant accumulation of 24(S)-hydroxycholesterol (24S-HC) and 27-hydroxy-cholesterol (27-HC) in addition to the cholesterol synthesis intermediates, desmosterol, lanosterol and lathosterol (PubMed:19633360, PubMed:18039927). 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 (PubMed:17916878) (By similarity). Belongs to the ABC transporter superfamily. ABCG family. Eye pigment precursor importer (TC 3.A.1.204) subfamily.