Early detection is a harder problem. Early on, when the tumour is small, there is not as much ctDNA to detect. The women Illumina identified as having cancer were all late, not early stage.

To date, there is one company offering a blood test based on ctDNA for early cancer detection: Epigenomics began offering its test for colon cancer in 2016 based on detecting biochemical modification of a single gene.

But the dream being imagined by Grail and others is an inexpensive test, perhaps no more than $500, which could conceivably be given annually to those over a certain age, with a high chance of detecting many cancers at once with high accuracy (Grail hasn’t announced a final number but thinks it will be in the region of 10). It’s a test that all of us, if it works, could one day get. “The big studies are still to be done,” says Nitzan Rosenfeld, a researcher who studies ctDNA at the Cancer Research UK Cambridge Institute and a cofounder of the UK-based liquid biopsy company Inivata, “but there has been considerable progress.”