On page 675, McKay and colleagues describe a method for generating virtually unlimited supplies of dopaminergic and serotonergic neurons from mouse embryonic stem (ES) cells in vitro. By monitoring the expression of gene products from the midbrain and hindbrain regions of the brain that give rise to these specialized neurons, they identified the mitogenic and signaling molecules that promote their differentiation from ES progenitor cells. They expanded the population of these types of neurons and showed that they could indeed produce dopamine and serotonin. The ability to easily culture large populations of such functionally active neurons could provide an important tool for the study and treatment of neurodegenerative and psychiatric disorders.