RBSN (rabenosyn) is a conserved Rab4/Rab5 effector protein that functions as a critical regulator of early endocytic membrane trafficking and cargo sorting. Structurally, RBSN contains a FYVE domain that binds phosphatidylinositol 3-phosphate (PtdInsP3), enabling its translocation to early endosomes where it mediates homotypic endosome fusion and fusion with clathrin-coated vesicles 1. RBSN bridges interactions between Rab5 and the SNARE fusion machinery through Vps45, linking small GTPase activation to membrane fusion events 2. The protein functions as part of the FERARI tethering platform that coordinates Rab11-dependent recycling pathways at sorting endosomes, combining fusion activity with membrane fission through interactions with VPS45 and EHD1 3. Beyond endosomal recycling, RBSN promotes transferrin receptor recycling to the plasma membrane and lysosomal trafficking of cathepsin D from the Golgi 1. Recently, RBSN was identified as a GAP protein that suppresses non-small cell lung cancer metastasis by inhibiting CDC42 activity and filopodia formation 4. Mutations in RBSN cause distinct Mendelian disorders: separation-of-function FYVE domain variants impair PI3P binding and delay endosomal maturation without affecting recycling, causing muscle weakness and ophthalmoplegia 1, while other mutations result in intractable seizures and multi-organ dysfunction through defective endocytic trafficking 5.