In the field of clinical diagnosis the determination of specific antibodies against distinct structural or functional antigenic proteins of a given pathogen is the most commonly used diagnostic tool for the detection of infections. Although most of the established test systems still use natural antigen from different sources, the advent of nucleic acid engineering opens up the possibility of producing proteins of limited natural availability as well as designing novel proteins using in vitro mutagenesis techniques. Recombinant technology has already proven to be an excellent alternative for the production of specific antigens, that are able to improve sensitivity as well as specifity. In general, the production of recombinant antigens for diagnostic purposes is inexpensive compared to the use of purified natural antigens. The major problems associated with the setup of recombinant test systems are, of course, the identification of those antigens or antigenic determinants that guarantee a safe serological diagnosis (see Chapter 1 and Chapter 2 ) and the expression of these fragments with high efficiency.