Multicolor fluorescence in situ hybridization, a new promising molecular cytogenetic technique, permits the simultaneous detection several specific nucleic acid sequences in interphase as well as metaphase by using cohybridization of probes labeled with serveral fluorescent labels or label combinations. It provids a simple, fast, and reliable means to assess genetic instability in cancer. This approach is being applied generally in physical mapping,mutation research, tumor pathology and prenatal dignosis.