[Cancer screening fantasies].

The use of circulating tumour DNA to screen for cancer in asymptomatic individuals is the goal of Grail, initially set up by Illumina. This company is now raising a large amount of capital to develop this project on a much longer time scale than initially announced. Although it has access to cutting edge technology and is led by excellent scientists, the prospect for success in this application is rather bleak, mainly because of the extreme requirements for specificity in order to avoid overdiagnosis.