A mixture of ganglioside GM2 with C18:1 and C20:1 sphingoid backbones; induces activated T cell death at 150 μg/ml in vitro; serum levels are increased in patients with breast cancer or cholangiocarcinoma; levels are elevated in the brain of patients with Sandhoff disease; accumulates in the lysosomes of individuals with Tay-Sachs disease and GM2-activator deficiency, as well as in the CNS of patients with and animal models of mucopolysaccharide storage disorders and Niemann-Pick disease types A, C1, and C2