Molecular cytogenetics has become an integral part of the discipline of cytogenetics and is indispensable in accurate chromosomal analysis of leukemias. FISH is a powerful tool for detection of numerical and structural chromosomal aberrations, thus providing a complementary approach in cases with a normal or failed cytogenetic result. In spite of continual technical developments in cytogenetic methodology, Some samples fail to produce a successful result because of low mitotic index or poor chromosome morphology, FISH has overcome these limitations, it has produced major improvements in the sensitivity and reliability of cytogenetic analysis for both research and routine applications, FISH has proven to be a powerful methodological approach to visualize specific nucleic acid sequences directly within the morphological context of the cell. The method involves hybridization of labeled probes to denatured target chromatin that been fixed to a microscopic slides, followed by visualization using standard (immuno) cytochemical procedures