Abstract: Any change (mutation) in the normal linear order of a chromosome or change in the typical chromosome number for a cell or organism. As genome sequencing is now revealing the linear order of chromosomes at the DNA level, even changes as small as the addition or deletion of a single DNA base pair could, in principle, be considered a chromosome abnormality. However, in practice, most changes of this type will be too small to be detectable using light microscopy, which is by far the most common technique used to evaluate chromosome structure. Therefore, the term chromosome aberration usually refers to large changes (?1% of chromosome length) in chromosome structure that are visible using light microscopic techniques. Since chromosomes from most multicellular eukaryotic species are composed of more than 107 DNA base pairs, changes of at least hundreds of thousands (105) of DNA base pairs would usually be required in order to be visible as a chromosomal aberration