GRM3 and schizophrenia: Within the full schizophrenia sample, monomeric mGlu3 was negatively correlated with age at onset of illness (N = 70; R = − 0.321, p < 0.01); however this correlation was weaker and non-significant in the sub-sample used for case-control comparisons (N = 46; R = − 0.171, p = 0.25).