Patients with advanced microsatellite unstable (MSI-H) colorectal cancer will be vaccinated with three so called frame shift peptides (FSPs), AIM2(-1), HT001(-1) and TAF1B(-1) combined with Montanide® ISA-51 VG. By this, an immune response directed against MSI-induced FSPs that are shared by the majority of MSI-H colorectal cancers can be induced. The aim is to show that vaccination against MSI-induced FSPs is safe and can induce or enhance immune responses against MSI-H colorectal cancer-associated antigens.