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For 150 years, Down's Syndrome has constituted the archetypal mental disability, easily recognisable by distinct facial anomalies and physical stigmata. In a narrow medical sense, Down's syndrome is a common disorder caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British asylum medical superintendent who described the syndrome as Mongolism in a series of lectures in 1866. In 1959, the disorder was identified as achromosome 21 trisomy by the French paediatrician and geneticist J--eacute--;r--ocirc--;me Lejeune and has since been known