Data were available on the prevalence of SPG4 from 13 studies in 10 countries [1, 6, 18, 19, 21–23, 28, 29, 31, 32, 34, 35], where the frequency of SPG4 relative to HSP ranged from 18% in Greece [29] to 95% in Italy-Sardinia [22]. The gene discussed is SPAST; the disease is hereditary spastic paraplegia.