Deficits in another enzyme in the ganglioside biosynthesis pathway, the GM2/GD2 synthase encoded by the B4GALNT1 gene, cause rare hereditary spastic paraplegia accompanied by intellectual and motor disabilities [63], concurrent with severe motor deficits, progressive dysmyelination, and axonal degeneration shown in B4galnt1-null mice [64,65,66]. The gene discussed is B4GALNT1; the disease is hereditary spastic paraplegia.