Cloning and identification of differentially expressed genes or proteins is helpful not only for finding the functions of genes and proteins, but also for discovery of the mechanism of some diseases. Some methods have been developed for screening differentially expressed genes, such as differential display RT-PCR (DDRT-PCR), subtractive hybridization (SH), DNA chip technique, and serial analysis of gene expression (SAGE). In subtractive hybridization, there have advanced three improved methods which include representational difference analysis (RDA), suppression subtractive hybridization (SSH), and full-length-gene-obtainable subtractive hybridization. For obtaining differentially expressed proteins, scientists have only two choices so far. One is two-dimentional gel electrophoresis. The other is phage display antibody repertoire library technique. Since all of the methods above have their own advantages and disadvantages, they should be used according to different needs.