In complex forms, the associated signs may be subtle but important indicators of the mutated gene, such as cerebellar atrophy or cerebellar ataxia with optic atrophy in SPG7, developmental delay and short stature in SPG20 (Troyer syndrome), dysarthria, distal amyotrophy, premature aging and cognitive decline in SPG21 (Mast syndrome), peripheral neuropathy and abnormal skin and hair pigmentation in SPG23 (Lison syndrome) (Table 2). This evidence concerns the gene SPG7 and Leber hereditary optic neuropathy.