JPMorgan Chase & Co. and Citigroup Inc. are expanding in "jumbo" mortgages used to buy the most expensive homes, helping revive a market that shriveled amid a three-year jump in homeowner defaults.

JPMorgan resumed buying new jumbo loans made by other lenders this month, after halting purchases in March, spokesman Tom Kelly said. Borrowers must have checking accounts with the bank, he said.

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