ATL1 and Alzheimer disease: The most common types are SPG4, which accounts for 40% of all autosomal dominant (AD) HSP; SPG3A, caused by a pathogenic variant in ATL1, which is the second most common type of AD HSP (accounting for 10–15% of all AD HSP) and is the leading cause of early-onset autosomal dominant HSP (occurring in >75% of individuals in this category); SPG30, caused by a pathogenic variant in KIF1A; and SPG31, caused by a pathogenic variant in REEP1 [81,82,83,84].