The mouse midbrain-hindbrain constriction is centrally involved in patterning of the midbrain and anterior hindbrain (cerebellum), as revealed by recent genetic studies using mice and embryological studies in chick (reviewed in [1,2]). This region can act as an organizer region to induce midbrain and cerebellar development. Genes such as Engrailed-1, Pax-2 and Pax-5, which are expressed in the embryonic cells that will form the midbrain and the cerebellum, are required for development of these regions. Fate-mapping experiments at early somite stages in chick have revealed that the cerebellar primordium is located both anterior and posterior to the midbrain-hindbrain constriction, whereas midbrain precursors lie more anteriorly. Fate mapping in mice has been complicated by the inaccessibility of the postimplantation embryo. Here, we report the use of a new in vivo approach involving the Cre-IoxP site-specific recombination system [3] to map the fate of cells in the mouse midbrain-hindbrain constriction. We show that cells originating in the mouse dorsal midbrain-hindbrain constriction during embryonic days 9-12 contribute significantly to the medial cerebellum and colliculi. Our data demonstrate the feasibility of using a recombinase-based lineage-tracing system for fate mapping in the mouse.