A large number of our genes are preferentially expressed from one allele due to regulatory polymorphisms occuring both in cis- and in trans-. This unequal expression contributes to susceptibility to common diseases, including cancer. Our main aim is to use this concept to develop new methods to study predisposition to breast cancer. Additionally, we are investigating the role that this imbalance can play in tumour biology, namely in the effect of coding mutations and treatment response.