Advances in molecular genetics in the past decade enabled us to analyze the cause of mendelian disorders at molecular level and a variety of mutations, not only in point mutations and deletion in exons but also in those occurred in regulatory elements or in RNA processing have been precisely identified. Such a variety of mutations may constitute variable clinical manifestations even in the simple mendelian disorders. On the other hand, pathogenesis of common diseases is much complicated and remains greatly to be elucidated. However, if we could use the strategies applied in the past few years for mendelian disorders, it seems to be not difficult to approach them. It is recommended to categorize a certain disease into subgroups for distinguishing their heterogenous phenotypes by clinical, biochemical and other properties. Owing to the success in making a subgroup (FAB classification), many subtype-specific translocations were found in leukemia, and then, rearrangement of relevant genes is also being shown. The best example is seen in chronic myelocytic leukemia. Since rearrangement of ABL and BCR was shown and both genes were cloned, detection of minimal residual diseases after intensive treatment became possible at 10(-6) level using RT-PCR technique. Recently developed interphase cytogenetics using FISH has visualized Ph1 translocation in metaphase cells and also in round nuclei, suggesting a potential use in monitoring the effect of certain drugs during treatment. Furthermore, very selective targeting therapy is being devised using antisense DNA.