Identifying the spinal circuits controlling locomotion is critical for unravelling the mechanisms controlling the production of gaits. Development of the circuits governing left-right coordination relies on axon guidance molecules such as ephrins and netrins. To date, no other class of proteins have been shown to play a role during this process. Here we have analyzed hop mice, which walk with a characteristic hopping gait using their hindlimbs in synchrony. Fictive locomotion experiments suggest that a local defect in the ventral spinal cord contributes to the aberrant locomotor phenotype. Hop mutant spinal cords had severe morphological defects, including the absence of the ventral midline and a poorly defined border between white and grey matter. The hop mice represent the first model where, exclusively found in the lumbar domain, the left and right components of the central pattern generators (CPGs) are fused with a synchronous hindlimb gait as a functional consequence. These defects were associated with abnormal developmental processes, including a misplaced notochord and reduced induction of ventral progenitor domains. Whereas the underlying mutation in hop mice has been suggested to lie within the Ttc26 gene, other genes in close vicinity have been associated with gait defects. Mouse embryos carrying a CRISPR replicated point mutation within Ttc26 displayed an identical morphological phenotype. Thus, our data suggest that the assembly of the lumbar CPG network is dependent on fully functional TTC26 protein.Significance statementOur work reveals novel developmental defects in hop mice affecting nervous system development and the assembly of local locomotor circuits. The hop mouse mutant appeared spontaneously in the 1960s but the underlying cause of the unnatural synchronously hopping gate has not yet been revealed. Altered functionality in hop mutant mice origins from an early developmental defect in the lumbar spinal cord, resulting in a fused ventral midline. The hop mouse represents an animal model harboring a very particular locomotor style that can help us understand the assembly and properties of locomotor networks and how a they function to coordinate motor behavior.