Human RAB5IF (RAB5-interacting protein) is a key effector protein of the Rab5 GTPase, primarily localized to early endosomes, where it serves as an essential functional and marker protein. By binding to activated Rab5, it participates in regulating processes such as endosome recognition, fusion, transport, and maturation, making it a core molecule in the study of clathrin-mediated endocytosis and vesicular transport. This protein plays a crucial role in maintaining endosomal dynamics and receptor sorting, while also influencing the spatiotemporal signaling of pathways such as those involving growth factor receptors. Studies suggest that functional abnormalities in RAB5IF may be associated with tumorigenesis and progression, yet its core research domains remain focused on organelle and subcellular labeling, cell biology, and intracellular signal transduction.