The availability of embryonic stem (ES) 1 cell lines has revolutionized biology by enhancing our ability to manipulate the genome and by providing model systems to examine cellular differentiation. These lines, derived from the inner cell mass of preimplantation blastocysts (1, 2), exhibit three particularly useful properties. First, they are totipotent; ES cells maintained in culture remain competent to contribute to all tissues, including germ cells, after introduction into mouse blastocysts (3). Second, targeted mutations in ES cells may be created readily through homologous recombination (4, 5) and introduced into the mouse germ line to study their effects in vivo. Third, wild-type or mutant ES cells can differentiate in vitro to form a variety of cell types in a sequence that recapitulates the first stages of murine embryogenesis. In vitro differentiation of ES cells offers a unique approach to examine events that occur during embryonic development and complements gene knockout studies in whole animals. Upon withdrawal of leukemia inhibitory factor and stromal contact, ES cells form embryoid bodies (EBs), spherical aggregates of differentiated cell types (Table I) which appear in a well-defined temporal pattern. Hematopoiesis within EBs has been studied most extensively (6–12), although vasculogenesis (6, 13, 14), myogenesis (6, 15–20), and development of neuronal-like cells (21) have also been observed.