Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of ACI Worldwide (Nasdaq: ACIW) must feel on top of the world today, enjoying gains of as much as 13% after the company announced an update to its proposed acquisition of S1 Corp. (Nasdaq: SONE).

So what: The two companies announced that the U.S. Department of Justice has closed its investigation related to the proposed merger, slightly ahead of the expiration of ACI's exchange offer a week from today. The news clears the way for the merger to proceed.

Now what: ACI's proposed acquisition includes $360 million in cash and 5.8 million of its shares and was initially proposed back in October. S1 shareholders can choose to receive either $10 per share or 0.3148 shares of ACI for each S1 share currently held, subject to proration and limits. S1's board recommends that all S1 shareholders tender all of their shares according to the offer.

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Fool contributor Evan Niu holds no position in any company mentioned. Click here to see his holdings and a short bio. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.