While significant progress has been made in the diagnosis and treatment of cancer, several major issues remain unresolved. First, only a minority of patients respond to most forms of (chemo)therapy. It is generally believed that this poor responsiveness to drugs has its origin, at least in part, in the molecular heterogeneity of cancer. This heterogeneity requires the use of biomarkers to stratify patients having seemingly similar tumors according to their likely responses to specific cancer therapies. To identify such biomarkers, we will use large-scale genetic screens to identify genes that are causally involved in controlling responses to cancer drugs. Such genes are likely biomarkers of drug responsiveness in the clinic. Availability of such drug response biomarkers will facilitate a more personalized therapy choice for each individual patient. A second major deficit in effective cancer therapy is the lack of sufficient highly selective drug targets. The large-scale cancer genome re-sequencing efforts already indicate that there is a paucity of druggable genes that are consistently mutated in cancer and the same holds true for genes that are consistently over-expressed in cancer. Hence, there is an urgent need for innovative drug targets that have a similar cancer-selectivity as the genes that are specifically mutated or over-expressed in cancer. In this project, we will use large-scale loss of function genetic screens, exploiting the concept of synthetic lethality , to identify genes whose inactivation is selectively toxic to cells having a defined cancer-specific genetic alteration. Drugs against these targets will be highly cancer-selective, as their activity hinges on the presence of a specific genetic defect, which is only present in the cancer cell.