Book excerpt: PCR, developed at Cetus Corporation/USA by Henry A. Erlich, Kary Mullis and Randall K. Saiki, is a very simple method for amplifying nucleic acids in vitro. The realization of this idea bases on the repetition of a set of three different temperatures and yields an increase of the target structure up to a factor of 106 to 1012. Therefore, this technique is predisposed for safe analysis and characterization of DNA and RNA sequences of interest, even where the starting amount of material is enormously small. Because of its sensitivity, speed and versatility this method is particularly suitable for investigations of oncogenes, tumor associated translocations, retroviral sequences, lymphokines and mainly the broad field of degenerative and inflammatory diseases of nervous system. PCR seems to be the technique which could overcome the two most important problems in that field: very small amount of material combined with the necessity of rapid diagnostic procedures in inflammatory infections. "PCR topics" will give an actual overview of basic and applied research fields on usage of polymerase chain reaction. All contributions to this book have been presented at an international congress on "Usage of Polymerase chain reaction in genetic and infectious diseases" which took place in june 1990 in Berlin. The editors wish to thank all participants for their contributions. We offer our thanks and gratitude to our coworkers and especially to our technical assistents Barbara Trampenau, Mirjana Wiirdemann and Hannelore Leonhard.