Book excerpt: The 9th volume of the series contains 11 validated and tested analytical methods and a collection of 6 LightCycler assissted real time methods with which the genotype and phenotype of a person can be determined with regard to the polymorphism of certain enzymes. The methods and enzymes selected here represent the current state fo the art in this field. These analytical methods are accompanied by chapters avout the biochemical basis of the susceptibility markers and by recommendations from the DFG Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area on how physicians should evaluate the analytical results.