RAB39B is a small GTPase that regulates intracellular membrane trafficking and plays critical roles in neuronal function and homeostasis. As a member of the RAB family, RAB39B cycles between GDP-bound inactive and GTP-bound active forms to control vesicular transport processes 1. The protein is involved in multiple cellular pathways including autophagy regulation, where it controls trafficking of Atg9-positive vesicles from the soma to synapses, thereby regulating synaptic protein turnover 2. RAB39B also participates in glutamate receptor trafficking and α-synuclein homeostasis 3. Loss-of-function mutations in RAB39B cause X-linked intellectual disability and early-onset Parkinson's disease 14. In neurodegeneration, RAB39B dysfunction leads to impaired dopamine vesicular transmission and disrupted α-synuclein clearance, contributing to protein aggregation 4. Studies in dementia with Lewy bodies show RAB39B redistribution and sequestration within amyloid plaques and Lewy bodies, with reduced cytoplasmic availability that may facilitate pathological aggregation 5. The protein's role in PI3K-AKT-mTOR signaling also affects neural progenitor cell proliferation, linking its dysfunction to macrocephaly and autism spectrum disorders 1. These findings establish RAB39B as a crucial regulator of neuronal membrane trafficking with significant implications for neurodevelopmental and neurodegenerative diseases.