In cancer, liquid biopsies such as plasma are an extremely attractive resource for the discovery of novel biomarkers. Because plasma collection doesn't require invasive procedures, it can be obtained serially and may help clinicians to improve or facilitate diagnosis, therapeutic decision making or therapy response monitoring. Various studies have indicated that plasma contains various RNA molecules. RNA molecules can be expressed in a highly-tissue specific manner. By combining this knowledge with RNA expression profiles of cancer patient plasma samples, this project aims to gain insights in the origin of plasma RNA and use plasma RNA profiles as a window into the activity of different tissues (including the tumor itself) in cancer patients.