Reuters is reporting that the FTC is looking to block the Express Scripts / Medco merger. This deal had a greater than $11 billion cash component. This is a big negative for the overall market if it turns out to be true. For the same reason that an $11 billion cash deal is positive for the market, the cancelling of a deal is negative. If the deal is called off arbs will sell the shares they bought into the market, removing liquidity.
If the deal is called off it would be a positive for my long suffering Walgreen's position. Express Scripts has been trying to strong arm Walgreen's into accepting significantly less than they receive from other PBMs. Express Scripts bargaining power will be greatly diminished if this deal is called off.