<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>A syndrome characterized by the presence of three complete copies of genetic material for chromosome 21, instead of the normal two. It leads to a variety of abnormalities that include mental retardation, macroglossia, microgenia, epicanthic eyelids, and a single transverse palmar crease.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>