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 Marshall & Ilsley (NYSE: MI) went "cha-ching!" today, rising as much as 21% after Canada's Bank of Montreal (NYSE: BMO) agreed to buy the Wisconsin-based bank.

So what: Bank of Montreal was looking to expand its U.S. presence, while M&I was still on somewhat shaky ground following the financial crisis and recession, so the deal seems to make sense from both sides. Of course, due to the fact that M&I is selling out at a book value multiple of just a bit above 0.6, some M&I shareholders may complain that the takeout price was too low. Officially, BMO agreed to exchange each share of M&I stock for 0.1257 shares of BMO stock which, before today's drop in BMO shares, valued M&I's shares at $7.75 each.

Now what: Depending on how BMO shares trade, there still is -- as of this writing -- more room for M&I shares to gain before hitting acquisition price. The fact that shares are trading below the offered price suggests that traders don't see strong potential for another buyer to come out of the woodwork and bid for M&I.

Looking at the broader banking sector, there were a gaggle of regional banks -- including SunTrust (NYSE: STI), Regions Financial (NYSE: RF), and KeyCorp (NYSE: KEY) -- that were up on the news. Investors are getting excited about a potential wave of M&A activity in the banking sector.

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