<n0:ComplexDefinition xmlns:n0="http://ncicb.nci.nih.gov/xml/owl/EVS/ComplexProperties.xsd#"><n0:def-definition>A constellation of symptoms that occur as a result of the presence of a complete third copy of the 18th chromosome. Characteristics include profound mental retardation and severe malformations. Individuals with this syndrome rarely live past one year.</n0:def-definition><n0:def-source>NCI</n0:def-source></n0:ComplexDefinition>