There is an internationally agreed system for describing the banding pattern of chromosomes, such that if an abnormality is accurately described in one laboratory then it can be recognized in another. This is known as the ISCN, the International System for Human Cytogenetic Nomenclature. Since its first appearance in 1965, it has been tried, tested, and modified, and the 1995 edition remains the standard version in current use (1 ). This is an essential reference for the definition of cytogenetic abnormalities. Within the ISCN are the formal descriptions of how to describe chromosome bands and abnormalities. There is also a schematic representation of the human karyotype, and several illustrations of karyotypes of normal chromosomes, stained in different ways. Every cytogeneticist needs to become familiar with the correct way of describing chromosomes and their abnormalities.
