Jérôme Jean Louis Marie Lejeune was born on June 13, 1926, in Montrouge, France, into a deeply Catholic family. Early on he felt drawn to medicine and, after completing his studies in Paris, joined the genetics research team under Professor Raymond Turpin in 1952. There he encountered children with what was then called “mongolism” (now known as Down syndrome) and the families caring for them, and resolved to dedicate his scientific career to their welfare and to understanding the root causes of their condition.

In 1958, Lejeune made a groundbreaking discovery: he identified that Down syndrome was caused by an extra copy of chromosome 21 (trisomy 21), thus establishing for the first time a genetic basis for intellectual disability. Over his career he also described other chromosomal anomalies, held the first chair of fundamental genetics at the Paris Faculty of Medicine, and received numerous international awards, including the Kennedy Foundation Award in 1962 and the William Allan Award in 1969.

Yet Lejeune’s legacy extended far beyond science: as a committed Catholic and believer in the dignity of every human life “from conception to natural death,” he became a prominent voice in bioethics, challenging the abortion of fetuses diagnosed with chromosomal conditions and helping to found the Pontifical Academy for Life, of which he was the first president in 1994. He died on April 3, 1994, in Paris after battling cancer, and on January 21, 2021, Pope Francis declared him Venerable.