SPAST and hereditary spastic paraplegia: In contrast to the situation in humans, where heterozygous spastin mutations are sufficient to cause HSP, we saw no frank axonal swellings or convincing locomotor phenotype in mice heterozygous for the spastin N384K-expressing allele, although in the 4 month old animals there was a trend towards several gait metrics for heterozygous animals being intermediate between wild-type and homozygous mutant values.