Spastin and P60-katanin are two distinct microtubule-severing proteins. Autosomal dominant mutations in the SPG4 locus corresponding to spastin are the most common cause of hereditary spastic paraplegia (HSP), a neurodegenerative disease that afflicts the adult corticospinal tracts. Here we sought to evaluate whether SPG4-based HSP is best understood as a "loss-of-function" disease. Using various rat tissues, we found that P60-katanin levels are much higher than spastin levels during development. In the adult, P60-katanin levels plunge dramatically but spastin levels decline only slightly. Quantitative data of spastin expression in specific regions of the nervous system failed to reveal any obvious explanation for the selective sensitivity of adult corticospinal tracts to loss of spastin activity. An alternative explanation relates to the fact that the mammalian spastin gene has two start codons, resulting in a 616 amino acid protein called M1 and a slightly shorter protein called M85. We found that M1 is almost absent from developing neurons and most adult neurons but comprises 20-25% of the spastin in the adult spinal cord, the location of the axons that degenerate during HSP. Experimental expression in cultured neurons of a short dysfunctional M1 polypeptide (but not a short dysfunctional M85 peptide) is deleterious to normal axonal growth. In squid axoplasm, the M1 peptide dramatically inhibits fast axonal transport, whereas the M85 peptide does not. These results are consistent with a "gain-of-function" mechanism underlying HSP wherein spastin mutations produce a cytotoxic protein in the case of M1 but not M85.