Genetic testing in patients with cancer; new developments About 5% of patients with cancer have a causative germline mutation. When a germline mutation is detected, this may have major implications for treatment and follow-up of the patient, as well as for relatives who are at risk of carrying the mutation. Increasingly, DNA-testing of tumor tissue is being performed to identify potential druggable targets, aiming at personalized medicine. Both germline testing and tissue testing may have consequences for the patient, for treatment and for family members. Currently there is a trend towards mainstreaming of genetic testing, which implies that treating physicians will increasingly be the ones to order DNA tests. This implies that they need to be aware of the (family) consequences and pitfalls of genetic testing. It calls for close collaboration between clinical genetics and regional treating physicians, and adequate referral of patients with abnormal DNA results and those with other clues for a genetic predisposition. The aim being optimal tailored treatment for each patient and adequate cancer prevention for their relatives.