Such conditions include HSP with thin corpus callosum (SPG11/15 mutations), SPG35/FA2H-associated HSP, Troyer syndrome (SPG20) and Mast syndrome (SPG21), SPG26/B4GALNT1, SPG30/KIF1A, SPG39/PNPLA6 and SPG46/GBA2. The gene discussed is B4GALNT1; the disease is mast syndrome.