NEW YORK (CNN/Money) - Online broker E*Trade Group Inc. said Monday it is offering a "portable mortgage" that allows customers to take a mortgage with today's low rates with them when they move to a new home.

E*Trade introduces a portable mortgage plan that allows customers to take a mortgage with them when they move. CNNfn's Mary Snow reports. Play video (Real or Windows Media)

Mortgage rates sank to record lows again last week and the news about E*Trade's new mortgage product initially drove its stock higher Monday though the shares turned lower at midday.

The E*Trade program allows borrowers who finance a home at current low rates to apply the terms of the loan to a new residence, rather than paying off the existing loan and applying for a new loan at potentially higher market interest rates.

E*Trade said the product, which it called "Mortgage on the Move," would be available for a limited time.

It's not clear how many other lenders will follow E*Trade's lead.

Most mortgages in the United States are sold to investors in the secondary market, and a non-traditional loan such as the E*Trade portable loan could not be sold, which may limit its appeal.

E*Trade is still a relatively small player in the mortgage business, which the industry group the Mortgage Bankers Association estimates will write $3 trillion in home loans this year.

The loan also may not work out for homeowners who move to a more expensive house and need a bigger loan to make the purchase. And demand for the portable loans could fall sharply if mortgage rates start climbing.

E*Trade, which started as purely an online broker, has been expanding its financial service offerings in recent years as individual investors pulled back from the bear stock market.

Banking income for the firm, which was less than half its brokerage revenue a year ago, jumped to 82 percent of brokerage revenue in the first quarter.

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During that period, Menlo Park, Calif.-based E*Trade said it generated $2.5 billion in mortgage loans and that it had $1.2 billion more in the pipeline at the end of the quarter.

E*Trade (ET: down $0.27 to $7.85, Research, Estimates) stock rose in early trading but later turned lower.