ST8SIA2 and psychiatric disorder: The involvement of ST8SIA2 fits with a polyfactorial model of susceptibility for mental illness, where defects in the development of brain connectivity early in life, caused by inheritance of risk variants in ST8SIA2 which reduce glycosylation of NCAM resulting in axonal misguidance and reduced neuronal plasticity, may render the brain more susceptible to secondary genetic and environmental insults later in life.