Art Cashin said investors need to carefully watch the yield on the U.S. 10-year note "as a proxy for whether there's problems in Ukraine (or) problems anywhere else," because that is what's driving the stock market.
Cashin, UBS' director of floor operations at the NYSE, told CNBC's Bob Pisani around midday that "the stock market today has been absolutely hypnotized by the 10-year. It's almost tick for tick. The yield goes up and stocks say, 'OK, less risk' and they go up, too... That's what we saw at the open. They opened up a little over 100 points. They wound up 55, all in direct response to that 10-year yield."
Cashin notes, however, that when he sent out his morning note by email this morning he got back "out of office" responses from about 30 percent of the people on his list. That's an indication many on Wall Street are taking the holiday-shortened week off, which will probably reduce trading volume.
—By CNBC's Alex Crippen. Follow him on Twitter