Identification of differentially expressed genes is one of the major challenges in molecular biology. Several techniques allow the cloning of such sequences. However, methods such as RNA subtraction or differential hybridization are time-consuming and require large amounts of mRNA. Recently, a new approach has successfully been developed: Differential-Display Reverse Transcription-PCR (DDRT-PCR). This technique has been proven to be highly effective in identifying sequences that are differentially expressed in various cell types. The most striking advantage is, however, that only nanograms of total RNA are sufficient. Thus every mRNA species expressed in the cell system can be investigated, even those present at very low levels.