MON1A is a guanine exchange factor (GEF) that plays a critical role in vesicular trafficking and cellular homeostasis. Working in concert with CCZ1 and C18orf8, MON1A catalyzes the exchange of GDP for GTP on the small GTPase RAB7, converting it from an inactive to active state 1. This activation is essential for directing RAB7 recruitment to autophagosomal and lysosomal membranes 1, and facilitating late endosome-lysosome trafficking and cholesterol homeostasis through NPC1-dependent lysosomal cholesterol export 2. MON1A localizes to secretory lysosomes through interaction with the V-ATPase a3 isoform, which recruits the MON1A-CCZ1 complex to these compartments 3. Beyond autophagy, MON1A functions broadly in protein trafficking through the secretory apparatus, including ferroportin trafficking to macrophage surfaces 4, and regulates SNARE complex assembly during autophagosome maturation 5. Disease relevance was recently established when MON1A mutations were identified as novel causal variants in congenital diarrhea and enteropathy (CODE), a severe pediatric disorder 6, suggesting MON1A dysfunction impairs intestinal epithelial protein trafficking or barrier function. The structural basis of MON1A's GEF mechanism involves direct interaction with RAB7's phosphate-binding loop 1, providing a framework for understanding disease mechanisms and therapeutic development.