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I'm looking at 30Y 4.0% Freddie Mac MBS prices as of yesterday, approximately 104:10. Meanwhile, 30Y FRM Freddie Mac quotes are 4.39%. Why are the MBS priced at such a premium, even though the coupon is below the interest rate? Does it have to do with prepayment assumptions? Could anyone please shed some light? Thank you!

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Here are some general contributing factors. I could be more specific, but you haven't given specifics about which Freddie, 30-year, 4% MBS price you are quoting. I'm going to assume it's the front-month TBA price. I'm also going to assume the 4.39% rate is the PMMS rate.

The PMMS is the primary market rate (what the borrower pays). The investor doesn't receive the full primary rate. The investor receives the coupon.

Freddie takes a portion of the interest as payment for guaranteeing the loan against default (the "guarantee fee" or "gfee"). And investors aren't interested in collecting payments, dealing with escrow, etc, so they pay a servicer to do that (the "service fee", or "base service fee"). These two fees can easily reduce the interest rate the investor receives by 50 basis points or more.

As you guessed, the remainder of the difference between the PMMS and the current coupon MBS is largely due to prepayment assumptions. For TBAs, the cheapest-to-deliver pool is also a contributing factor.

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