ABCG4 (ATP-binding cassette subfamily G member 4) is a half-transporter protein that functions primarily in cellular cholesterol efflux and lipid homeostasis. The protein consists of a single nucleotide-binding domain followed by six transmembrane segments and shows 72% identity to ABCG1 1. ABCG4 exhibits ATP-dependent transporter activity, functioning as both homodimers and heterodimers with ABCG1 2. The transporter mediates cholesterol efflux from cells to lipidated lipoproteins, working sequentially with ABCA1 to generate cholesterol-rich HDL particles 3. ABCG4 is highly expressed in brain tissue and shows site-specific expression in cerebral cortex and medulla 4. The gene is regulated by oxysterols and retinoids through LXR/RXR nuclear receptors, suggesting involvement in macrophage lipid homeostasis 1. In Alzheimer's disease research, ABCG4 suppresses γ-secretase activity and reduces amyloid-β production by altering γ-secretase distribution in membrane raft domains 5. The transporter also plays roles in megakaryocyte development, where its expression affects cell proliferation and differentiation 6. These findings indicate ABCG4's importance in neuronal cholesterol transport and potential neuroprotective functions.