Fannie Mae's issuance of MBS jumped 20% in December after a similar increase in November, according to new figures released by the mortgage giant. 

Fueled by refinancings, MBS issuance hit $76.3 billion in December, up from $63.7 billion in the prior month.

But Fannie also reported that commitments to purchase mortgages from its seller/servicers fell 16% from November to $51.4 billion -- which could signal the refi boom may be slowing somewhat. Then again, the yield on the benchmark 10-year Treasury this week was within 9 basis points of matching its all-time low.

Meanwhile, Fannie's single-family delinquency rate dropped below 4% for the first time since June 2009. The percentage of guaranteed loans 90 days or more past due fell 9 basis points from November to 3.91% in December.

The GSE's serious delinquency rate was stuck at 4% for the three preceding months.

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