Regencell Bioscience Holdings a small Hong Kong company, is grappling with a struggling business and a Justice Department investigation. Despite recurring losses, no revenue and no salable products, Regencell finished last week with a $15.5 billion stock-market value, up 50% since year-end and 126-fold since its initial public offering in 2021. For context, only 20 of the 261 companies in the Nasdaq Biotechnology Index had a greater market value; the index doesn’t include Regencell.