ATG9B is a transmembrane phospholipid scramblase that plays a critical role in autophagy by mediating autophagosomal membrane expansion 1. As one of two mammalian homologs of yeast Atg9, ATG9B shows tissue-specific expression patterns, particularly in placenta and pituitary gland, contrasting with the ubiquitous expression of ATG9A 2. The protein functions by cycling between the phagophore assembly site and cytoplasmic vesicle pools to supply membrane for growing autophagosomes 2. ATG9B's scramblase activity enables redistribution of phospholipids from cytoplasmic to luminal leaflets, driving membrane expansion during autophagosome formation 2. In cancer contexts, ATG9B maintains stem cell properties and is transcriptionally regulated by ASCL2 in gliomas, where high ASCL2-ATG9B axis expression correlates with poor prognosis and serves as a biomarker for autophagy inhibitor sensitivity 1. The protein is also involved in cholesterol homeostasis, being upregulated by SREBP-1 to promote lipophagy in glioblastoma cells 3. Additionally, ATG9B facilitates immune evasion mechanisms, as demonstrated by its role in cancer stem cell-derived oncosome formation that promotes immunosuppressive macrophage differentiation 4. Its expression can be modulated by viral oncoproteins like HPV16 E6/E7, linking it to cervical cancer pathogenesis 5.