SPG21 and hereditary spastic paraplegia: Another notable feature of the case described in this study is the presence of a thinning corpus callosum (TCC; Fig. 1A), which has been described as a subcategory of complex HSP, most commonly attributed to autosomal recessive mutations in SPG11 (spatacsin), with fewer cases caused by SPG21 (maspardin), SPG32, SPG15, and TUBB4A (13, 64).