The serotonin 5-HT(6) receptor (5-HT(6)R) is a promising target to improve cognitive symptoms of psychiatric diseases of neurodevelopmental origin, such as autism spectrum disorders and schizophrenia. However, its expression and localization at different stages of brain development remain largely unknown, due to the lack of specific antibodies to detect endogenous 5-HT(6)R. Here, we used transgenic mice expressing a GFP-tagged 5-HT(6)R under the control of its endogenous promoter (Knock-in) as...