Oligosaccharide domain of the monosialylated glycosphingolipid GM2 of a-series. The compound is present on neuronal cells and plays a key role in the regulation of dendritogenesis in cortical pyramidal neurons. In lysosomal storage disorders Tay-Sachs and Sandhoff disease, where hexosaminases A and B are deficient, GM2 ganglioside accumulates in the nervous system. GM2 is also overexpressed in melanomas and other tumours of neuro-ecto origin. Moreover, the sugar moiety of this ganglioside is a receptor allowing viral infection of cells with reovirus and rotavirus.