BACKGROUND: Skeletal muscle differentiation requires assembly of contractile 
proteins into organized myofibrils. The Drosophila ladybird homeobox gene (lad) 
functions in founder cells of the segmental border muscle to promote myoblast 
fusion and muscle shaping. Tetrapods have two homologous genes (Lbx). Lbx1 
functions in migration and/or proliferation of hypaxial myoblasts, whereas the 
function of Lbx2 is poorly understood.
RESULTS: To elucidate the role of Lbx in vertebrate myogenesis, we examined Lbx 
function in zebrafish. Zebrafish lbx2 transcripts appear in newly formed 
paraxial mesoderm and become restricted to adaxial cells, precursors of slow 
muscle. Slow muscles lose lbx2 expression as they differentiate, while a subset 
of differentiating fast muscle cells transiently expresses lbx2. Fin and hyoid 
muscle express lbx2 later. In contrast, lbx1b expression first appears lateral 
to the somites at late segmentation stages and is later restricted to fin 
muscle. Morpholino knockdown of Lbx1b and Lbx2 suppresses hypaxial muscle 
development. Moreover, knockdown of Lbx2 results in malformation of muscle 
fibers and reduced fusion of fast precursors, although no obvious effects on 
induction or specification are observed. Expression of myofilament genes, 
including actin and myosin, requires the engrailed repressor domain of Lbx2.
CONCLUSION: Our results elucidate a new function of Lbx2 as a regulator of 
myofibril formation.