Two main classes of glutamate receptors have been characterized, the ionotropic (iGluRs) and the metabotropic (mGluRs) glutamate receptors. In order to better understand the function of the latter, we have used the technique of gene targeting to generate mice in which the mGluR1 subtype has been inactivated. The disruption was carried out by homologous recombination in embryonic stem (ES) cells at the level of the seven-membrane domain, leaving the extracellular part of the receptor untouched. In addition, the reporter gene lacZ was inserted in frame with mGluR1 coding sequence within the second intracellular loop of the receptor. The transmission of the mutation to the germ line showed first that the fusion protein was functional and second that mGluR1 was inactivated. Therefore, the way homologous recombination was performed in ES cells demonstrated that gene replacement of mGluR1 by lacZ could be a powerful technique to disrupt a gene and at the same time study its endogenous expression.