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Summary: INTRODUCTION
The nerve cords of organisms as diverse as nematodes, flies
and humans embrace a common architecture with axial arrays
of motoneurones distributed along midline bundles of neuronal
processes (Hedgecock and Hall, 1990; Eisen, 1998; Jurata et
al., 2000). In each case, locomotion depends on the coordinated
activities of distinct classes of motoneurones, the interneurones
that regulate them and the muscles that they innervate. The
proper function of each neuronal subtype is defined by the
adoption of appropriate axonal trajectory, neurotransmitter
expression and synaptic connectivity. What are the
mechanisms that regulate these specific traits? Recent evidence
indicates that homeodomain (HD) proteins exercise key roles
in the specification of cell type identity in the motoneurone
circuit. Hierarchical cascades of interacting HD proteins
segregate the vertebrate spinal cord into distinct progenitor
domains (Briscoe et al., 2000; Muhr et al., 2001). Subsequently
expressed HD proteins may induce the differentiation of
specific interneurone and motoneurone subclasses within each
of these regions (Tanabe et al., 1998; Jessell, 2000). The roles
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