Circulating tumor DNA assays are becoming relevant for routine diagnostics, but many related aspects are yet unresolved. With this project, the investigators aim to develop pragmatic molecular diagnostic pathways of liquid biopsies relevant in advanced gastrointestinal malignancies with focus on clinical utility and sensible use of resources. They want to evaluate the ctDNA assays on a fully automated low-cost multiplex platform which is already implemented in routine molecular diagnostics of solid biopsies. The project will evaluate to what extent these ctDNA assays are relevant for clinical decision-making.