NEW YORK ( TheStreet) -- The Charles Schwab Corp. ( SCHW) was cut to "Neutral" from "Buy" by Goldman Sachs in a report published Wednesday that was bearish on the discount brokerage sector as a whole. In downgrading Schwab, Goldman analysts cited a decline in yields on residential mortgage backed securities (RMBS) issued by Fannie Mae ( FNMA)and Freddie Mac ( FMCC) following the Federal Reserve's announcement last week that it would buy long-dated RMBS in an effort to boost the housing market. So called "agency" RMBS make up 70% of Schwab's securities portfolio, according to Goldman, and the decline in yields may encourage pre-payments and "headwinds" in net interest margins. Net interest margins refers to the difference between a company's cost of capital and the rate it earns through lending. Goldman's analysts see Schwab outperforming E Trade Financial ( ETFC) and Ameritrade ( AMTD), but still believe "upside in the stock is limited." Shares of Schwab were down by 1.15% a few minutes after the opening bell Wednesday. Ameritrade and E Trade shares were also lower. -- Written by Dan Freed in New York. Follow this writer on Twitter.