- A defect in the ionotropic glutamate receptor 6 gene (GRIK2) is associated with autosomal recessive mental retardation. Motazacker, M.M., Rost, B.R., Hucho, T., Garshasbi, M., Kahrizi, K., Ullmann, R., Abedini, S.S., Nieh, S.E., Amini, S.H., Goswami, C., Tzschach, A., Jensen, L.R., Schmitz, D., Ropers, H.H., Najmabadi, H., Kuss, A.W. Am. J. Hum. Genet. (2007)