Numerous genes whose expression is controlled by complex regulatory networks are involved in the development and progression of each cancer,and those genes will be the key factors for determining each characteristic of the tumor. The recent development of DNA microarray and related technologies provides an opportunity to perform more detailed characterization( profiling) of individual tumor cells. Indeed, the gene expression profile of a tumor provides a unique molecular portrait or signature that can be correlated with clinical behavior and drug responsiveness. The development of personalized( or customized) medicine and molecularly targeted drugs is enthusiastically awaited based on the results of research examining genomic diversity such as SNPs( single nucleotide polymorphisms), gene expression profiling with DNA microarrays, and analysis of protein expression and interactions.