Abstract: The current invention relates to a method for selecting oligonucleotides able of identifying differences at the molecular level between a plurality of biological systems. This method has been named “STRIPGEN” for “Selective Technique for Rapid Identification of Proteins and GENes”. The STRIPGEN method involves selection of oligonucleotides having a higher affinity and specificity for target molecules of a first biological system and a lower affinity for binding target molecules of a second biological system. STRIPGEN is based on a sheme of positive selection over a first biological system and negative selection over a second biological system, and uses step-wise iterations of binding, separation and amplification. This method is very powerful since it can discriminate very subtle differences between biological systems of the same type such as between normal and abnormal cell types.