By some measurements, the market for commercial MBS backed by single-family rental units has been successful, drawing investors from Wall Street, the hedge fund community and overseas. But that success, to some degree, is beginning to worry the Treasury Department’s Office of Financial Research. Moreover, the OFR also is expressing anxieties about mortgage real estate investment trusts and repurchase agreements. As of September 2014, roughly $5 billion of single-family rental MBS had been issued...