In July 1958, whilst examining the chromosomes of a child suffering from "Down's syndrome", he discovered the existence of a chromosome too many on the 21st pair. For the first time in the world, a link was established between mental debility and a chromosomic aberration.
In 1964, he became the first professor of Fundamental Genetics at the Paris Medical Faculty.Whilst keeping himself readily available for the families of handicapped children who he treated, he took an active part in thousands of conferences world-wide.
In 1974, he became a member of the Pontifical Science Academy.
In 1981, he was elected as a member of the Academy of Moral and Political Sciences.
In 1983, he joined the National Medical Academy. He was made an honorary doctor, was granted membership or received awards from many other foreign academies, universities and learned societies.
In 1994, he was appointed President for life of the Pontifical Academy.
He died on 3rd April 1994, with the sad feeling of failing in his mission : "I was the doctor who was supposed to cure them and as I leave, I feel I am abandoning them."
Professor Lejeune received numerous awards for his work on chromosomic pathologies, among which :in 1962, the prestigious Kennedy prizein 1969, the William Allen Memorial Awardin 1993, the Griffuel prize, for his pioneering work on chromosomic anomalies in cancer