SUSD4 (sushi domain containing 4) is a complement regulatory protein that functions as a complement inhibitor through multiple mechanisms 1. The protein disrupts formation of the classical C3 convertase, with soluble isoform 3 inhibiting the classical complement pathway and membrane-bound isoform 1 inhibiting C3b deposition via both classical and alternative pathways 1. SUSD4 exhibits tissue-specific expression patterns, being highly expressed in brain white matter on oligodendrocytes/axons, photoreceptor outer segments of the eye, spinal cord, and testis 2. Beyond complement regulation, SUSD4 demonstrates tumor suppressor functions across multiple cancer types. In breast cancer, SUSD4 expression correlates with better patient prognosis and reduced tumor cell migration and invasion 3. The protein interacts with EGFR and initiates autophagy in an EGFR phosphorylation-independent manner, activating the LKB1-AMPKα1 pathway 4. In the nervous system, SUSD4 knockout mice exhibit motor performance defects, increased anxiety-like behaviors, and abnormal neuronal morphology including reduced dendritic spine density 5. SUSD4 deletions are associated with autism and Fryns syndrome, highlighting its importance in neurodevelopment 2. The protein represents a promising therapeutic target given its dual roles in immune regulation and cancer suppression.