HTR6 and Alzheimer disease: Serotonin type 6 receptor (5-HT6R) belongs to the family of G-protein-coupled receptors (GPCRs), which has emerged as a promising target for the treatment of cognitive decline associated with neurodegenerative (e.g., Alzheimer’s disease and Parkinson’s disease) and psychiatric disorders (e.g., depression and schizophrenia).