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Cytogenetic research has had a major impact on the field of medicine, especially in oncology and reproductive medicine, providing an insight into the frequency of chromosomal abnormalities that occur during gametogenesis, embryonic development, and tumor development (1). This is well emphasized by the continuing focus on genetic abnormalities that are associated with, as well as probably responsible for, tumor origin, tumor progression, spontaneous abortions, and congenital anomalies. However, information on recurrent chromosomal aberrations in solid tumors and in some hematological cancer is still limited. The growth of solid tumor in culture for cytogenetic analysis is poor and is compounded by low mitotic indices (2). Often the specimens are contaminated with bacterial and other microbial agents and might contain large regions of necrotic tissue. In addition, the major clone that does grow might not reflect its true representation in the tumor in vivo, where multiple subclones exist with complex chromosomal alterations, making identification of primary genetic changes difficult. Furthermore, solid-tumor metaphase chromosomes often have poor morphology (1). A newly described molecular-cytogenetic technique that does not rely on growth of the tumor in culture might well accelerate the rate at which perturbed chromosomal regions can be cytogenetically identified and molecular-genetically characterized in solid tumors (2). In the past decade, fluorescence in situ hybridization (FISH) has significantly improved the cytogenetic analysis of tumors. FISH utilizes specific chromosomal probes, usually composed of cloned fragments of DNA (3) (see Chapter 29). These probes will anneal only to their matching complementary DNA sequences on target chromosomes and can accurately detect and target one specific gene or chromosome region at a time. [ABSTRACT FROM AUTHOR]