In anticipation of the eventual Fed withdrawal from mortgage-backed securities purchases, MBS-related exchange traded funds have tracked lower and are now testing their summer lows.

The iShares MBS Bond ETF (NYSEArca: MBB) fell 3.4% over the past three months. The minutes of the Fed’s July meeting released Wednesday showed the central bank will likely taper its bond and mortgage purchases before the end of 2013.

“In our view, from the stock perspective, September tapering of MBS purchases seems to be an eminently viable option for the Fed,” Bank of America Merrill Lynch analysts said in a HousingWire report.

Under the current bond purchasing program, the Federal Reserve is purchasing around 50% of monthly gross issuances of mortgage debt, according to Morningstar analyst Timothy Strauts.

“Some quick back-of-the-envelope calculations suggest that up until May when mortgage rates and bond yields began to rise rapidly on the back of fears that asset purchases would be tapered prematurely, each $100 billion of MBS purchases reduced 30-year mortgage rates by 0.06 percentage points,” Capital Economics property economist Paul Diggle said in a separate HousingWire article.

The yields on 30-year fixed mortgage are currently hovering around 4.56%. [Investors Tap Mortgage Bond ETFs for Yield, Rate Protection]

The iShares MBB ETF tries to reflect the performance of the Barclasy U.S. MBS Index, which tracks agency, or government, issued securities by government-sponsored entities, such as Fannie Mae, Freddie Mac and Ginnie Mae. MBB has a 0.27% expense ratio, a 2.99% 30-day SEC yield and a 4.8 year effective duration. A longer duration translates to a greater decline if interest rates were to rise.