(1) C1q−/− mice had significantly lower infarct volume, improved neurological score, lower C3 deposition, and less leukocyte activation at 24 h. (2) C1q−/− mice also had lower cerebral atrophy and better performance on water maze up to 8 weeks after hypoxia-ischemia. The gene discussed is C3; the disease is Cerebral atrophy.