The step-by-step differentiation of embryonic cells into different types of neurons lays the foundation for our sensory responses, motor commands, and cognitive behaviors. Our research explores such differentiation programs in mammals using a combination of genetic, embryological, and molecular biological methods. While the generation of such neural diversity is a complex process culminating in the most sophisticated of wiring circuits, one simplifying approach is to start by tracking the specification, differentiation, and migration paths taken by specific sets of cells originating from primitive neuroectoderm. Towards this goal, our lab has generated a variety of recombinase-based transgenic tools for both descriptive and functional studies in mice. Using these tools we are able to define progenitor-progeny cell relationships, distinguish cell lineages based on molecular identity, and perturb these lineages in various ways to reveal cell function in the living mouse. We are applying these and other molecular genetic and genomic tools to study programs underlying the development and function of various neural systems in the brain stem, paying particular attention to the precerebellar system (with its central role in movement control and sensorimotor transformations), the serotonergic system (with its involvement in such disparate functions as sleep, arousal, homeostasis, pain, and depression), and the choroid plexus (as an organizing/patterning center during embryogenesis and as the source of cerebral spinal fluid).