Definition of Deletion

Deletion: Loss of a segment of DNA from a chromosome (and hence from the genome).

The first human chromosome deletion was detected in 1963 by Jerome Lejeune and his colleagues in Paris. They discovered loss of part of 5p, the short (p) arm of chromosome 5, in children with a hitherto-undescribed condition with mental retardation and multiple congenital anomalies (birth defects) they named the cri du chat (cat-cry) syndrome.

A chromosome deletion is the opposite of a chromosomal duplication.