DNA methylation is the most extensively studied mechanism of epigenetic gene regulation, a biological mechanism that will switch genes on and off. The universal methyl-group donor for DNA methylation is S-adenosylmethionine (SAM), which is provided by one-carbon metabolism. Methyl donors like methionine, folate, betaine and choline are micronutrients which are essential to the decent functioning of one-carbon metabolism. Aberrations in this pathway by deficiency or excess in any of these micronutrients, could affect the DNA methylation process. Improper alterations in DNA methylation patterns can affect normal patterns of gene expression or alter genome stability and thereby lead to an increased risk of cancer. Therefore, the objective of the present project is to examine the hypothesis that dietary intake of methyl-group donors involved in one-carbon metabolism may influence breast cancer risk through alterations in DNA methylation patterns. To investigate this hypothesis, the existing resources of the European Prospective Investigation into Cancer and nutrition (EPIC) study will be used to address the major gaps in the understanding of the role of methyl donor nutrients, DNA methylation and breast cancer development by applying a nutri-epigenetic approach. Simultaneously, an in vitro model will be developed to examine the effects of methyl- group donor exposure on epigenetic changes in breast cell lines. This can contribute to the understanding of the mechanisms of action.