The authors report a nucleotide substitution (c.1216A>G) in SPG4 (spastin) causing hereditary spastic paraplegia. This apparent missense mutation in the ATPase domain confers aberrant, in-frame splicing and results in destabilization of mutated transcript. Mutated protein is deficient in microtubule-severing activity but, unlike neighboring mutations, shows regular subcellular localization. The authors' data point to haploinsufficiency rather than a dominant negative effect as the disease-causing mechanism for this mutation.