S100A9 and schizophrenia: A key feature was upregulation of the calcium-binding heterodimer calprotectin (S100a8 and S100a9), a marker of microglial activity that is elevated in schizophrenia [48], multiple sclerosis [49] and Alzheimer’s disease [50], where it may contribute to amyloidogenesis [51] and activate TLR4 signalling [52].