On Wednesday, stock market investors saw renewed fears about the sustainability of the five-year-old bull market, as heightened tensions between Russia and Ukraine contradicted the hopes that the conflict would quietly fade after Russia's annexation of Crimea. In addition, a growing number of analysts believe that the stock market's valuation has become high, and that hit some stocks especially hard. Among the worst performers of the day were Exelixis (NASDAQ:EXEL), ReneSola (NYSE:SOL), and Capstone Turbine (NASDAQ:CPST).

Exelixis plunged 39% after the biotech said last night that an independent committee had recommended that its phase 3 trial of its prostate cancer drug cabozantinib continue. Investors had hoped that the results would be strong enough that the committee would recommend an early termination of the study, explaining the disappointment even though the press release didn't mention detailed information about the interim trial results. Cabozantinib has already gotten FDA approval for treatment of a form of thyroid cancer, but it's important for the company to get additional indications approved in order to maximize the value of the drug. Moreover, with warnings of sometimes-fatal hemorrhage and gastrointestinal perforations, Exelixis might have to deal with safety concerns even if the remainder of the trial goes well.

ReneSola dropped 14% following news that the U.S. Commerce Department had selected the solar company as one of the participants in its investigation of alleged dumping of Chinese solar products. In response, ReneSola said it would stop shipping its products to the U.S., risking a huge drop in its sales because of its heavy exposure to the U.S. solar market. With more than a quarter of its revenue having come from the U.S. during its most recent quarter, ReneSola badly needs a positive result from the Commerce Department investigation in order to keep up its growth at a critical time for the Chinese solar industry.

Source: Capstone Turbine.

Capstone Turbine fell 11% on another negative day for alternative energy companies, as Plug Power (NASDAQ:PLUG), Ballard Power Systems, and FuelCell Energy (NASDAQ:FCEL) were all down between 15% and 25% on the day. Capstone's microturbines use a different technology than the fuel-cell systems that Plug Power and FuelCell Energy offer, but many analysts see the companies as potential competitors serving the same prospective customer base, especially as they grow and seek to capture more of the available opportunity in the broader space. With Capstone having doubled since December before today's drop, it's clear that momentum investors are just as scared of its stock as they are of Plug and FuelCell Energy -- at least for today.