REP15 (RAB15 effector protein) is a GTPase effector that interacts with active, GTP-bound RAB15 and other Rab proteins including RAB3A-D and RAB34 1. Its primary function is regulating transferrin receptor recycling from the endocytic recycling compartment, a specialized compartment distinct from sorting endosomes 2. REP15 localizes specifically to endocytic recycling compartments and directly binds Rab15-GTP but not other Rab isoforms, enabling compartment-specific control of slow receptor recycling pathways 2. Beyond membrane trafficking, REP15 modulates downstream signaling including cell proliferation and migration 1. Clinically, REP15 dysregulation associates with colorectal cancer pathogenesis, identified as a prognosis-related driving gene in multi-omics analyses 3. Additionally, REP15 emerges as a candidate gene for Kallmann syndrome combined with intellectual disability when subject to dosage alterations 45, though causal roles require confirmation through targeted sequencing. The compartment-specific nature of REP15 function highlights mechanistically distinct endocytic recycling pathways and represents a potential therapeutic target in cancer and neurodevelopmental disorders.