GRM1 and paraneoplastic cerebellar degeneration: Based on the triggers of autoimmune process or the related anti-neuronal antibodies, ACA can be divided into several subtypes including paraneoplastic cerebellar degeneration (PCD), primary autoimmune cerebellar ataxia (PACA), glutamic acid decarboxylase (GAD) antibody-related cerebellar ataxia, metabotropic glutamate receptor 1 (mGluR1) antibody-related cerebellar ataxia, among others (1).