MON1B (MON1 vesicular trafficking associated B) is a guanyl-nucleotide exchange factor that functions as a component of the Mon1-Ccz1 complex in late endosomal trafficking. MON1B localizes to late endosomes and the cytoplasm, where it regulates vesicular docking and homotypic fusion of early endosomes 1. Specifically, MON1B is recruited to early endosome membranes by the adaptor protein Numb, and its presence is essential for proper tethering and fusion of paired endocytic vesicles 1. MON1B also participates in phagosome-lysosome fusion pathways, though it is not directly targeted by mycobacterial virulence factors 2. Clinically, MON1B has emerged as a significant prognostic marker in cancer. A single nucleotide polymorphism in an NF-κB binding site near MON1B (rs284924) independently predicts prostate cancer-specific mortality in patients receiving androgen-deprivation therapy 3. Furthermore, MON1B expression is elevated in colon cancer tissues and cell lines, and MON1B knockdown inhibits cancer cell proliferation, migration, and invasion while increasing tissue inhibitor of metalloproteinase-2 expression and decreasing matrix metalloproteinase-2/9 and NF-κB signaling 4. These findings suggest MON1B may represent both a mechanistic regulator of endosomal trafficking and a potential therapeutic target in cancer.