The determination of gene expression levels in normal tissue is necessary for the analysis and interpretation of results of gene profiling studies in pathological samples. With the real-time reverse transcription-PCR technique, which enables one to detect the amplification rate during the process, assessment of the amount of gene transcript is fast and accurate. The most important problem in this type of analysis is the variability in the amount of genetic material between samples, caused mostly by changes in the efficiency of mRNA isolation and reverse transcription. Therefore, a reference gene to normalize sample variations is required. Quantification of the mRNA of the target and the reference gene in the sample ensures that the changes in transcript levels will influence both genes equally. To be used as a reference, a gene should show stable, unregulated expression in the analyzed sample type. Housekeeping genes (HKGs) fulfill this criterion and they are used for normalization purposes in most expression studies. However, transcript levels of HKGs can vary between different types of tissue (normal and pathological samples) and under different treatment conditions (drugs and chemicals). The aim of this study was to show the differences and the factors which can influence housekeeping gene expressions.