DAI
or ETH.
See token list page for supported token symbols1000
[{"token": {"price": 169.59,"name": "WETH"},"rates": {"dYdX": {"lend": {"rate": 0.5338044182707921},"borrow": {"rate": 2.37960754661304},"term": "1 block","name": "dYdX"},...}}, ...]