Cytogenetics has recently become an important component of laboratory examination methods used for diagnosing oncological and non-oncological disorders. Limitations of this method (low sensitivity, ineffective cultivations) lead to the establishment of new methods eliminating drawbacks of this classical microscopic technique. Fluorescent in situ hybridisation (FISH), sometimes also called molecular cytogenetics, largely supplements and partly even replaces karyotyping. The most sensitive methods used in genetics laboratories are methods of molecular genetics. Using these methods we are able to detect point mutations and DNA changes within the size of just a few base pairs. These methods are mostly used for diagnosing of hematooncological diseases and also for therapy monitoring.