It may not be the biggest threat to the mortgage industry, but bond investors say it could impact the American homebuyer bottom line.

According to the Structured Finance Industry Group, proposed amendments to Rule 4210 from the Financial Industry Regulatory Authority would establish margin requirements and close-out requirements in the To Be Announced sector of the mortgage securities market.

That's the secondary market for trading Fannie Mae and Freddie Mac bonds. It's highly liquid, and also the second largest of its kind in the world. So FINRA is keen to add controls if it maintains stability in the market.

According to an email from SFIG, FINRA stated that the “TBA market is one of the few markets where the exchange of margin has not been a common practice, thereby creating a potential risk from counterparty exposure.”

However, the Association of Institutional Investors responded recently, arguing that, if adopted, the Rule 4210 amendments will lead to lower liquidity, and “reduced liquidity in the agency MBS market, in particular the TBA market, will cause a meaningful increase in hedging costs for mortgage originators which, in turn, will mean higher borrowing costs for American homebuyers,” according to SFIG.

The news comes on the heels of the revelation that the cost of closing mortgages is officially astronomical.

The comment period closed last week. FINRA regulates the TBA market and will have final say.