GABBR1 and early-onset autosomal dominant Alzheimer disease: The GABAB receptor is a heterodimer with subunits encoded by GABBR1 and GABBR2. GABBR1 is encoded in the Major Histocompatibility Complex (MHC) extended class I region (6p21.3) [14], an area of the MHC associated with MS, MG, Alzheimer's disease, schizophrenia, narcolepsy, epilepsy, and RA.