Currently, in order to escape some of the restrictions that Congress and Treasury have put in place to stop inversions, the shareholders of the U.S. company must own less than 60 percent of the combined company. Pfizer's shareholders would own 56 percent of the combined company, for example. But that is in part because Allergen has completed previous acquisitions of U.S. companies that have increased its size. The biggest was when the former Actavis bought Allergan.