By the closing bell on Monday, the S&P 500 Index (SNPINDEX:^GSPC) had logged a fifth straight month of gains. Those gains helped the benchmark index post an impressive second-quarter performance, as the S&P tacked on 5% in the last three months alone. But while the S&P's quarter was solid, its day wasn't so great, as components like Valero Energy (NYSE:VLO), Intuitive Surgical (NASDAQ:ISRG), and Allergan (NYSE:AGN) dragged down the benchmark. The S&P ended slightly lower, shedding 0.7 points today, or less than 0.1%, to finish at 1,960.

Valero Energy finished the day as the single-worst performing stock in the group of 500, shedding 3% on Monday. Valero shareholders have reason to be bearish: the U.S. government, in an unforeseen and rather sudden move, announced that it will allow some U.S. companies to export unrefined oil abroad, overturning a 40-year policy in the process. This wasn't just bad news for Valero Energy, but for all U.S. oil refiners, who had previously been guaranteed business from oil companies seeking to export -- before doing so they'd need to refine it into another product.

Da Vinci Xi Surgical Arms Thumb

"da Vinci Xi with Splayed Arms." Source: Intuitive Surgical

Shares of robotic surgery company Intuitive Surgical lost 2.7% on Monday. The slump comes after three consecutive days of increasing gains; last week its newest da Vinci robot, the da Vinci Xi, received the Conformité Européenne Mark, or CE Mark, which will allow the da Vinci Xi to officially be marketed in countries that require the CE Mark as a regulatory thumbs-up. Intuitive Surgical's new system is already approved by the FDA to market the product in the U.S.

The last of today's laggards is also seeing the effects of regulation. Allergan shares tumbled 2.7% Monday after the FDA sent the company a second complete response letter regarding Semprana (formerly Levadex), an acute migrane treatment. In effect this pushes back the earliest date that Allergan can see Semprana officially approved by the FDA until next year. The FDA is reportedly concerned about the aerosol delivery method of the drug, though Allergan has defended Semprana and even suggested it may have data to address the FDA's concerns.

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John Divine has no position in any stocks mentioned. You can follow him on Twitter, @divinebizkid, and on Motley Fool CAPS, @TMFDivine.

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