Biotechnological inventions during the 20th century have resulted in a wide range of approaches for explorations in the functional genomics field. Microarray technology is one of the recent advances which have provided us with snapshots of which genes are expressed in cells of various tissues and diseases. Methods to obtain reliable microarray data are continuously being developed and improved to meet the demands of biological researchers. In this thesis microarrays have been used to investigate gene expression patterns in cancer research. Four studies in three different areas were carried out covering adrenocortical tumors, p53 target genes and a comparison of RNA amplification methods. Adrenocortical tumours are among the most common tumours with an incidence of 7-9%. Malignancy of these tumors is rare. Distinction between malignant and benign tumours is often difficult to establish which makes an improvement of diagnostic approaches important. To elucidate biological processes in adrenocortical tumour development and to examine if there is a molecular signature associated with malignancy, microarray analysis was performed on 29 adrenocortical tumors and four normal specimens. It was possible to classify malignant and benign samples based on the entire expression profile. A number of potential biomarkers was identified which will be further evaluated. P53 is a gene which is mutated in 50% of all cancers. Functional p53 is a transcription factor which is activated upon cellular stress and DNA damage. Target genes are mainly involved in cell cycle arrest and apoptosis. In solid tumors cells are stressed by hypoxia. To examine which target genes p53 activate under hypoxic conditions a microarray study of the cell lines HCT116p53+/+ and HCT116p53-/- was performed. A set of novel potential p53 target genes was identified while many known target genes were found to be not transcriptionally activated during hypoxia. Follow up which was focused on how p53 affected hypoxia induced apoptosis showed that the death receptor Fas was critical. When small amounts of tissue are available, amplification of the transcript population is necessary for microarray analysis. A new strategy for amplification based on PCR was evaluated and compared to a commercial in vitro transcription protocol. Both protocols produced reliable results. Advantages with the PCR based method are a lower cost and a high flexibility due to compatibility with both sense and antisense strand microarrays. Keywords: adrenocortical tumour, apoptosis, cancer, classification, gene expression, microarray, p53, RNA amplification / QC 20100907