Specifying the target identity of motoneurons

In the vertebrate spinal cord, motoneurons are clustered into longitudinal columns in agreement with the targets they innervate. Motoneurons within each column acquire properties early in development that ensure their axons navigate to appropriate targets, but how this target identity is specified is unknown. Recently, Tsuchida et al. described the expression of putative regulatory genes within motor columns in the chicken spinal cord. Combinations of LIM-family homeobox genes differentially mark columns that project to distinct target groups. Expression precedes column formation and axon outgrowth, making these genes candidates for specifying the target identity of motoneuron groups.