Locomotor behavior in mammals requires a complex pattern of muscle activation. Neural networks, known as central pattern generators (CPGs) and located entirely within the spinal cord, are responsible for generating much of the timing and pattern required for locomotor movements. Historically, identification of interneuronal components of the locomotor CPG in walking mammals has proven troublesome, primarily because of the difficulty in identifying functionally homogeneous groups of neurons in the spinal cord. Recently, a molecular approach has been used to identify populations of genetically similar interneurons based on the expression of transcription factors early in embryonic development. Preliminary work on these cell populations has shown that many comprise essential components of the locomotor CPG. Here I identify populations of genetically-defined interneurons that are candidate "first-order" cells of this neural network, potentially responsible for generating the locomotor rhythm in the mammalian spinal cord. Identification of the cell population(s) responsible for this key function will provide valuable insight into the structure and function of the locomotor CPG and could potentially lay the groundwork for the development of strategies aimed at regenerating motor pathways following injury to the spinal cord.