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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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