SPAST and hereditary spastic paraplegia: However, our findings on biomarker levels in SPG4 might be representative for the whole group of HSP patients with a pure phenotype, as (i) SPG4 is by far the most frequent genotype within this group and (ii) genotypes with similar clinical patterns can be expected to show similar biomarker levels due to a consistent extent of affected fiber tracts and rate of neuroaxonal decay [8,30].