Homeobox genes encode DNA-binding transcription factors that can regulate axial patterning, segment or cell identity, and proliferation. The classical homeobox genes are organised in clusters and determine the basic body plan of animals

Homeobox genes encode DNA-binding transcription factors that can regulate axial patterning, segment or cell identity, and proliferation. The classical homeobox genes are organised in clusters and determine the basic body plan of animals (Maconochie et al., 1996). In addition, homeobox genes exist that are clustered or dispersed in the genome. The two ladybird genes of Drosophila belong to a cluster of homeobox genes involved in mesoderm development and are located on the same chromosome, but some distance away, from the classical homeobox gene clusters (Jagla et al., 1994). They function in cell fate decisions of somatic muscle and cardiac cells, and participate in development of epidermal cells (Jagla et al., 1997a,b, 1998). Ladybird-like genes were recently also identified in mammals. The first member characterised, Lbx1, is expressed in developing skeletal muscle and the nervous system (Jagla et al., 1995). During muscle development, Lbx1 expression is restricted to migrating hypaxial muscle precursor cells and provides thus an excellent marker for the identification of this lineage (Jagla et al., 1995; Dietrich et al., 1998, 1999). The restricted expression of Lbx1 indicates that it regulates properties that are specific for the migratory lineage of muscle precursors. Ordered and directed cell migration is an essential developmental mechanism, by which precursor cells reach targets distal from their site of origin. Muscle precursors, neural crest cells, primordial germ cells and other cell types move over large distances, using characteristic paths. Attractive and repulsive cues are thought to direct the migrating cells. The elucidation of the molecular nature of such cues has been an area of active research. Cell-cell and cellmatrix adhesion molecules play important roles in migration in vitro and in vivo, and the corresponding ligands are thought to provide the appropriate substratum for migration (Montell, 1999). Ligands recognised by tyrosine kinase receptors can provide positive or negative cues for cell migration in mice and C. elegans; mutations that affect signalling components of these receptors can result in mislocation and misrouting of cells (Donovan, 1994; Robinson et al., 1997; Wehrle-Haller and Weston, 1997; Britsch et al., 1998; Chen and Stern, 1998; Montell, 1999). Netrins that function in the guidance of developing axons can also provide cues that direct the migration of muscle precursor cells in C. elegans (Culotti and Merz, 1998). In vertebrates, migrating myogenic precursor cells are generated from the lateral dermomyotome, a derivative of the somite (Chevallier et al., 1977; Christ et al., 1977). The lateral dermomyotome in the trunk gives rise to hypaxial muscles (superficially, laterally and ventrally located muscles), which are formed by a migratory and a non-migratory cell population (reviewed by Christ and Ordahl, 1995). The migratory 437 Development 127, 437-445 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV2467