ABCA9 is an ATP-binding cassette transporter primarily localized to the endoplasmic reticulum (ER) that functions as a cholesterol transporter involved in lipid homeostasis 1. The protein consists of two transmembrane domains and two nucleotide binding folds characteristic of full-size ABC transporters 2. ABCA9 expression is induced during monocyte differentiation and is cholesterol-responsive, suggesting a role in macrophage lipid homeostasis 2. Mechanistically, ABCA9 promotes cholesterol accumulation in the ER and suppresses sterol-regulatory element binding protein-2 (SREBP-2) translocation from the ER to the nucleus, thereby inhibiting cholesterol synthesis gene expression 1. In endothelial cells, ABCA9 activates AKT signaling to promote cell proliferation and angiogenesis 3. Clinically, ABCA9 shows significant disease relevance. Expression is downregulated in breast cancers and correlates with poor prognosis 1. High ABCA9 expression associates with reduced survival in serous ovarian cancer patients 4. Conversely, ABCA9 restoration in breast cancer cells suppresses cell proliferation and colony formation via SREBP-2 inhibition 1, suggesting potential therapeutic value. ABCA9 is also downregulated in lung adenocarcinoma and correlates with TP53 mutations 5. Additionally, ABCA9 is implicated in papillary thyroid cancer progression through miRNA-mediated regulation 6.