As companies pile into debt markets, the difference between swap rates and Treasury yields has gone negative.

As big banks shrink their trading in U.S. Treasuries, this crucial market is becoming less and less transparent.

U.S. regulators said to have decided to offer banks a lucrative concession that would remove collateral requirements on swaps trades with overseas affiliates.

Four-week T-bills will likely continue to offer negative yields as U.S. Treasury cuts issuance to keep federal government's spending under debt limit.