"DoorDash exists today to empower those like my Mom who came here with a dream to make it on their own," Xu wrote in a letter accompanying the company's IPO prospectus filing with the SEC. This article has been updated with Xu's comments on CNBC after DoorDash stock started trading on Wednesday morning. Don't miss: Amex Blue Cash Preferred is offering an elevated welcome bonus for a limited time Founder of 1-800-GOT-JUNK? dropped out of college to haul junk — now, he's eyeing a billion-dollar business GT Dave started brewing kombucha in his bedroom and turned it into a nearly $1 billion empire