Jérôme Lejeune was a French pediatrician and researcher who lived from 1926 to 1994. He achieved international prominence for his work in genetics, specifically for establishing the link between Down syndrome and an extra copy of chromosome 21. This groundbreaking finding provided the first evidence that certain intellectual disabilities were caused by chromosomal abnormalities, effectively founding the field of cytogenetics. Beyond his laboratory work, he was a dedicated Catholic who later became known for his vocal opposition to prenatal testing used for eugenic purposes.