Neurons are the basic signaling components of the brain. Consequently, a fundamental part of any attempt to understand how the brain works as a whole is the investigation of functionally distinct neuronal cell types. Toward this end, immunohistochemical markers have emerged as one of the most valuable tools available to neuroscientists. Using antibodies against various cell components, investigators are able to identify cells expressing a neuronal phenotype and, moreover, collect information regarding their morphological characteristics and expression of specific proteins. Other approaches exists to define neuronal cell types. For example, physiological optical tagging sequencing (PhOTseq) relies on the initial GCaMP calcium imaging labeling of neurons through activation and then the permanent labeling of specific neurons with photoactivatable mCherry