When Express Scripts Inc. got blindsided this week by the nation's largest drugstore chain, Walgreen Co. —which publicly threatened to end their multibillion-dollar relationship — the episode marked an epic clash in the pharmaceutical industry. Walgreen officials, in announcing they had canceled contract negotiations with the St. Louis-based pharmacy benefit manager, pronounced themselves 'surprised" that Express Scripts had the temerity to drive such a hard bargain with "the largest retail provider in their pharmacy network." The reaction from Express Scripts? Whatever— customers can just take their Express Scripts benefit cards to competitors literally down the block. "On average, another pharmacy within the Express Scripts network is within one-half mile of a Walgreens pharmacy," taunted a company release, while careful to also emphasize an interest in continuing talks. Wall Street responded by driving down Walgreen stock — while Express Scripts' shares inched up, apparently unaffected.