On March 21st, 2009, World Down Syndrome Day will mark the 50th
anniversary of the publication of Professor Jerome Lejeune's discovery
of the extra copy of chromosome 21 which causes Down Syndrome. Now known as trisomy-21 because of the third copy of the 21st
chromosome, Down Syndrome was first described by the English doctor,
John Langdon Down, in 1866 but until Professor Lejeune's publication in
1959, the ultimate cause of Down Syndrome was unknown.