To cite another important example, brain-derived neurotrophic factor (BDNF) has pleiotropic effects on neuronal development, differentiation, synaptogenesis and the synaptic plasticity that underlies circuit formation and cognitive function, and has been found to be decreased in brains and serum from patients with schizophrenia, anxiety and AD (Carlino et al., 2013; Lu et al., 2013; Mitew et al., 2013). Here, BDNF is linked to Alzheimer disease.