SUSD4, also known as sushi domain-containing protein 4, is a hypothetical cell surface protein whose tissue distribution and function are completely unknown. SUSD4 is detectable in murine brains, eyes, spinal cords, and testis but not other tissues. In brains, SUSD4 is highly expressed in the white matter on oligodendrocytes/axons, and in eyes, it is exclusively expressed on the photoreceptor outer segments. In in vitro complement assays, SUSD4 augments the alternative but not the classical pathway of complement activation at the C3 convertase step. SUSD4 deficiency may cause autism or Fryns syndrome, both of which are genetic diseases with severe abnormal neurological development and/or functions.