We aimed to investigate (in an existing research cohort of adults with DS) whether a peripheral marker of oxidative stress, specifically a change over time in lipid peroxidation (measured with urinary iPF2alpha) is associated with cognitive decline in people with DS, and whether this is related to anti-oxidative enzyme levels, including Cu/Zn Superoxide dismutase (SOD1). The gene discussed is SOD1; the disease is Mental deterioration.