Ipilimumab, a monoclonal antibody targeting CTLA-4, is approved for the treatment of metastatic melanoma and significantly increases median overall survival. However, use of this drug is associated with immune related adverse events (IRAEs) like colitis, hepatitis, dermatitis, alveolitis and hypophysitis in 10-40% of the patients. In general IRAEs are manageable by cessation of ipilimumab in combination with treatment with corticosteroids or TNF-alpha blockade but they can be severe or even life-threatening. In addition, treatment with ipilimumab is expensive. Because of the high costs and the potential serious toxicity of ipilimumab, it is of great importance to identify biomarkers that correlate with clinical activity and can be used to select patients that will benefit from CTLA-4 blockade therapy.