Bitcoin may be taking another step toward mainstream adoption, CNBC has learned.

    For the first time, customers of some U.S. banks will soon be able to buy, hold and sell bitcoin through their existing accounts, according to crypto custody firm NYDIG. The company, a subsidiary of $10 billion New York-based asset manager Stone Ridge, has partnered with fintech giant Fidelity National Information Services to enable U.S. banks to offer bitcoin…