F8A3 (coagulation factor VIII associated 3) is an X-chrX-encoded RAB5A effector molecule involved in early endosomal trafficking. As part of the HTT-F8A1/F8A2/F8A3-RAB5A complex, F8A3 mediates recruitment of HTT protein to early endosomes and regulates their interaction with the cytoskeleton by promoting actin filament association while inhibiting microtubule engagement, thereby reducing endosome motility. F8A3 is located in a genomic region adjacent to autism susceptibility loci and shows differential expression in Rett syndrome, a severe X-linked neurodevelopmental disorder. The gene is part of the F8 (coagulation factor VIII) locus structure, which is clinically significant for hemophilia A diagnosis, as F8A1, F8A2, and F8A3 comprise inversion breakpoint regions used in molecular detection of severe hemophilia A mutations. F8A3's role in intracellular trafficking suggests potential relevance to neurological function, though direct mechanistic links to disease pathogenesis remain incompletely characterized. Its proximity to X-chromosome X elements affecting nearby genes like SPRY3 indicates possible coordinated control of endosomal and neuronal signaling pathways.