Gene amplification is a copy number increase of a restricted region of a chromosome arm. It is prevalent in some tumors and is associated with overexpression of the amplified gene(s). Amplified DNA can be organized as extrachromosomal elements, as repeated units at a single locus or scattered throughout the genome. Common chromosomal fragile sites, defects in DNA replication or telomere dysfunction might promote amplification. Some regions of amplification are complex, yet elements of the pattern are reproduced in different tumor types. A genetic basis for amplification is suggested by its relative frequency in some tumor subtypes, and its occurrence in "early" preneoplastic lesions. Clinically, amplification has prognostic and diagnostic usefulness, and is a mechanism of acquired drug resistance.