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In oncology, it is often difficult to ascertain the chromosomal make-up of a tumor by conventional banding methods even where dependable metaphases can be obtained, since the chromosomes often exhibits a poor banding pattern. In spite of this, particular chromosome abnormalities, especially translocations, characteristic for certain types of solid tumors or leukemias, have been established by banding methods and more recently in combination with FISH. Such specific alterations have in some cases correlated with deregulation of translocated genes (Iida et al.

In a historic perspective it was a major breakthrough in 1956 when it was established that man has 46 human chromosomes (Fig. 5A) allowing for a new type of method to screen the whole genome. Refinements of the chromosome preparation techniques allowed a grouping of the chromosomes by size (Fig. 5B) and when banding techniques in the early 1970s were established, it became possible to characterize each individual chromosome (Fig. 5C,D). The development of FISH-techniques (Fig. 4) was encouraged by an ever-increasing number of identified but yet uncharacterized chromosomal aberrations such as simple or complex translocations and the presence of marker and derivative chromosomes.

2) (Fig. 3), and aniridia/Wilms tumor (delllp13) as examples and more will probably prevail. However, it should be noted that for the listed syndromes microde1etions account for a varying degree of the etiological factors. In for example Prader-Willi and Angelman syndromes, uniparental disomy and imprinting are also important etiological factors that cannot be detected by FISH. The developments of arm specific probes for each chromosome increased the possibilities for identifying and characterizing intra-chromosomal structural abnormalities that by conventional banding methods can be difficult to ascertain (Fig.