US-852 (Changsha mandarin [Citrus reticulata Blanco] x English Large Flowered Trifoliate Orange [Poncirus trifoliata]), originally created in 1965 by Dr. Joe Furr at Indio, California. It was released for use in Florida as a rootstock around 1999, but later discontinued for a number of reasons.

Cold hardy to 5F. Fruit is tart and quite edible. Downside is that it is seedy (around 15-20 seeds per fruit) and fruit adheres to the peel strongly. Still, I find the 852 citrandarin to be quite tasty and makes for a nice tart lemonade.