U.S. stocks opened slightly lower Monday as Wall Street digests one of the largest mergers in history. Pfizer (PFE) and Allergan (AGN) are combining in a $160 billion deal. This is also the biggest merger yet using a controversial tax-saving strategy. Allergan shareholders would receive 11.3 shares of the newly combined company for each of their existing shares, while Pfizer investors will get one share of the new company for each of their shares. The new combination would retain Allergan's legal and tax domicile in Ireland. Pfizer would have its global operational headquarters in New York and its principal executive offices in Ireland. Pfizer, the maker of erectile dysfunction medication Viagra and cholesterol-lowering drug Lipitor, and Allergan, whose brands include cosmetic medication Botox, predicted the combined company would have more than $25 billion in operating cash flow starting in 2018.