To identify the source(s) of C1q in the brain, the C1qa gene was selectively inactivated in the microglia or Thy-1+ neurons in both wild type mice and a mouse model of Alzheimer’s disease (AD), and C1q synthesis assessed by immunohistochemistry, QPCR, and western blot analysis. This evidence concerns the gene C1QA and early-onset autosomal dominant Alzheimer disease.