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 HealthSpring (NYSE: HS) soared more than 30% on Monday after insurer Cigna (NYSE: CI) said it would buy the health-maintenance specialist for about $3.8 billion.

So what: The all-cash deal values HealthSpring at $55 per share and represents a whopping 37% premium to its Friday closing price. Cigna is making the move to boost its presence in the increasingly attractive Medicare segment, and judging by its stock's small gain today, investors seem to be satisfied with the strategy, as well as the purchase price.

Now what: While HealthSpring shares are likely all sprung out, Cigna might just be getting started. Cigna's Medicare customer count is set to triple thanks to the deal, with management expecting earnings to increase in just the first full year of operations. Of course, for enterprising investors looking for the next big buyout, Medicare specialists like Humana (NYSE: HUM) and WellCare (NYSE: WCG) -- both of which are also rallying on the news -- may be worth looking into.

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