Following the initial reports of the cloning of a DNA fragment, modern molecular genetics found immediate practical application in molecular analysis and diagnosis of human diseases.Genomic DNA, RNA, nucleic acids from archival specimen or cloned DNA may be starting materials for gene analysis.In extrame cases complete analysis can be performed on the DNA from a single cell or a few microdissected chromosome fragments, or on RNA from only few cells. Many variations of the basic analytical procedures have nov been described and applied to a range of medical disciplines. These include, the polymerase chain reaction (PCR), which rapid detection of fast or slow growing microorganisms and viruses, such as mycobacteria and HIV, the detection of minimal residual diseases in leukaemia and in HLA typing. The analysis of archival and forensic material has applications in forensic pathology and evolutionary biology. PCR technique has also established a central role in the human genome project.