First, major classes of cells—e.g., neurons, astrocytes, ependymal cells, oligodendrocytes, vascular cells, and immune cells—are distinguished by large sets of class-specific genes that implement the specific function of each class of cells. For example, neurons share an extensive gene program involving synaptic, cytoskeletal, and ion channel genes, while oligodendrocytes express gene programs required for generating myelin. Multiple levels of hierarchical subdivision exist within these classes; for example, within neurons, neurotransmitter phenotype showed a modular and highly regulated pattern of expression.
