Testing for C9orf72, the most common hereditary cause of ALS in Caucasian populations, is reasonable to consider in the workup for HSP and has identified mutations in 0.9–2.9% of PLS patients as compared to identification in 30–40% of familial ALS patients and 5–10% of sporadic ALS cases [43,50,51]. Here, C9orf72 is linked to amyotrophic lateral sclerosis.