GRM1 and cerebellar ataxia: PLCβ4-KO mice show severe cerebellar phenotypes, similar to those shown by mGluR1-KO mice (Aiba et al., 1994; Kano et al., 1997; Ichise et al., 2000), such as persistent innervation of PCs by multiple climbing fibers, impaired long-term depression at parallel fiber–PC synapses, and severe ataxia (Kim et al., 1997; Kano et al., 1998; Miyata et al., 2001).