An Reyners is a medical oncologist. Among others, she is specialized in the treatment of patients with sarcoma. Gastrointestinal stromal tumours (GISTs) are a rare variety of sarcoma. GISTs are characterized by an oncogenic driver mutation in cKIT or PDGFRalpha. This mutation can be detected by analysis of ctDNA. Reyners and colleagues have set up a Dutch biodatabank containing longitudinal, prospective collected ctDNA samples of hundreds of patients with GIST. With a special designed drop off ddPCR known KIT exon 11 mutations of patients with GIST can be detected in serum. Moreover, based on ctDNA analysis of serum the diagnosis of GIST has been made in a few patients in whom a tumour biopsy was not feasible at that moment.