The elusive dream is that a simple blood test could detect a small tumor growing in your body, giving doctors time to cure you before it’s too late. Today, scientists at GRAIL, a biotechnology company based in Menlo Park, California, that has drawn more than a billion dollars in investment, announced progress toward that goal here at the annual meeting of the American Association for Cancer Research. Using full-genome sequencing to analyze DNA shed into the blood by dying tumor cells, the widely watched company saw evidence of cancer in 65% of a group of patients already known to have early disease.