What happened

The stock market had a mixed day on Monday, with the S&P 500 finishing modestly lower and the Nasdaq rising slightly. The financial sector largely lagged the market, but Citigroup (C 2.82%) was the worst performer out of the big banks. The stock finished the day nearly 5% lower.

So what

The main catalyst for the move is Citigroup's recently reported exposure to Russia. Citi is by far the most international of the big U.S. banks, with operations throughout the world. The bank has a substantial branch network in both Russia and Ukraine, and while it had previously announced plans to attempt a sale and exit of its Russian business, it hasn't yet done so.

Citigroup reports $2.9 billion in consumer and corporate loans in its Russian segment, as well as $1.5 billion in investment securities. It has $1 billion in cash at Russian financial institutions, and $1.8 billion in reverse repurchase agreements. Plus, other Citigroup units around the world have $1.6 billion in Russian exposure. Add all of this up, as well as certain other assets on its balance sheet, and Citigroup has nearly $10 billion in exposure to Russia -- not exactly an ideal situation.

Bank sign on a building.

Image source: Getty Images.

Now what

For one thing, this is exposure, not losses. Not yet, anyway. While it could certainly result in substantial losses, there's too much uncertainty surrounding the Russian situation at this point to make a determination. And Citi didn't reveal how much of these assets, if any, were affected by sanctions imposed since the situation began.

What's more, keep in mind that Citigroup has about $2.29 trillion in total assets, so the Russian exposure represents about 0.4% of the total. To be sure, multibillion-dollar losses would sting, but Citigroup investors shouldn't panic because of this news.