Cancer detection startup Grail today filed to go public as the Silicon Valley biotech aims to raise more funding for its blood test. The company’s backers include Microsoft co-founder Bill Gates and Amazon CEO Jeff Bezos; both invested in a $100 million Series A round four years ago. Total funding is now north of $1.9 billion, including a $390 million Series D round in May. Grail does not have revenue or a commercially-available product. That reflects the traditional life cycle of biotech companies, which commonly invest in research and development for years before bringing products to market.