The project is mainly a collaboration between the University of Tromsø and INSERM, France. We will pool together information from the E3N cohort of about 100 000 French female teachers with the 150 000 participants in the Norwegian Women and Cancer study , NOWAC, both started in the early nineties. NOWAC is a representativ population based cohort study. In a nested case-control design we will look at breast cancer risk according to widely different hormonal compounds in the brands of hormonal replacement therapy used in the two countries. With more than 1000 and 1500 breast cancer cases, respectively, the statistical power will allow us to find a relative risk of 1.2. Further, we will perform genomewide micro array analysis of peripheral blood from a samp le of 100 French participants in order to compare the expression pattern with similar analysis of Norwegian participants in users and non-users of different brands of HRT. The expression analysis can be used for mechanistic studies of metabolism of exogen ous hormones. Lastly, in the NOVAC cohort we will use the unique prospective sub-cohort of about 40 000 women with a prospectively collected RNA sample (buffered in a PAXgene tube) to study the genome wide expression in relation to later breast cancer. Th e statistical power with 140 cases and 280 controls will give us a possibility to find a relative risk (fold change) of about 1.3. Follow-up of the E3N cohort is done every second year with repeated measurements of HR use. In NOWAC follow-up is by linkage to the Norwegian Cancer Registry and with updated exposure information each 5. year.