The rapidly growing field of Chemical Biology combines the power of synthetic chemistry with the spectrum of micro- and molecular biology techniques, thus important challenges in medicine can be addressed in revolutionary new ways. This innovative methodology allows problems associated with studying diseases and designing novel drug-like molecules to be tackled in a unique approach. A recent example is the combination of synthetic carbohydrate chemistry with parasite glycobiology that has led to a new candidate for a malaria vaccine. Besides infectious diseases, cancer is a major cause of death worldwide and in Europe 2.4 million cases of cancer are reported per year, 50% of which are fatal.In this proposal we present a chemical biology route to propel research in cancer biology forward so that new drug targets can be identified. This global aim will be achieved by using a multidisciplinary approach, which employs peptide synthesis, molecular biology, NMR spectroscopy, X-ray crystallography and other cutting-edge biophysical techniques.This project, which aims to synthesize different ER modified constructs for elucidating comprehensively the role of post-translational modifications in the development of breast cancer and resistance to tamoxifen, would be of high importance for advancing prevention, early detection, monitoring and treatment of the breast cancer disease.