In mouse models of Alzheimer’s disease (AD) (Hong et al. 2016; Shi et al. 2017) and frontotemporal dementia (FTD) (Lui et al. 2016), elevated levels of complement factors cause early synaptic loss which can be rescued by inhibition or deletion of C1q, C3 or CR3. The gene discussed is C3; the disease is frontotemporal dementia.