Synapsin III (SYN3) is a neuronal protein with dual roles in neurobiology and oncology. In the nervous system, SYN3 is involved in synaptogenesis and regulation of neurotransmitter release 1, functioning at synaptic vesicles to modulate synaptic transmission. In cancer biology, SYN3 uniquely acts as a tumor suppressor in glioblastoma stem cells (GSCs) by promoting neuronal-like transdifferentiation 1. Mechanistically, SYN3 enhances expression of Neuregulin 3, which competitively antagonizes JAG1-Notch1 signaling—a critical pathway maintaining GSC stemness 1. This disruption of Notch signaling reduces self-renewal capacity and drives GSCs toward differentiation. In preclinical glioblastoma models, AAV-mediated SYN3 overexpression significantly impeded tumor progression 1. Clinically, SYN3 has also served as a delivery enhancer: the polyamide compound Syn3 markedly potentiates intravesical adenoviral-mediated gene transfer to bladder epithelium 23, facilitating the FDA-approved nadofaragene firadenovec therapy for BCG-unresponsive bladder cancer 45. These divergent functions highlight SYN3's importance both as a developmental regulator suppressing cancer stemness and as a pharmaceutical excipient enhancing gene delivery.