This dissertation was aimed to describe significant changes on the level of messenger RNA upon the concentration modification of a feed component in animal diets. In addition, the gene expression analysis with high-density microarrays was established with differently fed animals. The book describes the detailed setup of the experiment from the animal diet over the tissue and sample preparation to the analysis of the gene expression data and the detection of regulatory transcription factors. The thesis gives important tips and tricks for genomics in animal nutrition with living animals. For the selenium experiment Sel-Plex(R)(Alltech Inc.), a selenized yeast, was supplemented to the broiler diet. Subsequently to the gene expression analysis in the liver, several altered protein activities and fatty acid concentration changes could be proven. In the second and third project alterations on the gene expression level could be shown upon the addition of mycotoxin-contaminated wheat and the myctoxin-binder Mycosorb(R) (Alltech Inc.). The differently expressed genes were assigned to previous results and several causal genes and transcription factors of detrimental effects were identified.