Cytogenetic studies were performed i n 311 children uith Down's syndrome, most of them(65%) from mothers under the age of 35. A t o t a l of 301, m r g o l s (Croup I) d i d not have any famil i a r history of t h i s syndrome, while t h e other 7 had two o r more affected close r e l a t i v e s (Group 11).
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