Analysis of polymorphic markers segregating in both intra- and interspecific crosses has allowed us to map the autosomal recessive mutation progressive motor neuronopathy (pmn) to mouse Chr 13. Although this mutation, based on its histological description, was reported as a model for infantile spinal muscular atrophy of the Werdnig-Hoffmann type, its localization to a region that is not homologous with human 5q makes it unlikely to be a homologue to SMA. The presence of the Extra-toe (Xt) locus in proximity to pmn will help in the detection of affected progenies before the onset of the degenerative process.