Friday's market action involved many of the same concerns that have plagued stock investors all week, including the planned referendum in Crimea on whether to break off from Ukraine and join Russia. Nervousness led to small declines for broad market measures, with the Dow posting its fifth straight loss. But some stocks suffered much more dramatic moves downward, with Plug Power (NASDAQ:PLUG), Aeropostale (NYSE:ARO), and Raptor Pharmaceuticals (NASDAQ:RPTP) among the worst performers of the day.

Plug Power fell 16% as the popular fuel-cell company suffered from analyst downgrades, reversing all of the stock's gains yesterday following Plug's positive earnings report. One analyst actually raised its price target along with its downgrade, arguing that at current prices, the stock's valuation is reasonable. Yet, the danger that another analyst identified was that it could take longer for potential customers to get the infrastructure in place to supply fuel-cell equipment with the hydrogen they need to operate. If that happens, then expected sales could end up getting deferred into future quarters or years, and that would slow Plug's growth rate and lead to a reversal of sentiment among growth-hungry momentum investors. The move also pulled down industry peers FuelCell Systems (NASDAQ:FCEL) and Ballard Power Systems (NASDAQ:BLDP), which dropped 9% and 5% respectively.

Aeropostale plunged 20% in the wake of the teen retailer's poor results for its fourth quarter. Sales crashed 16%, producing an adjusted loss in what is always a key quarter for the retail industry. Guidance for those losses to double during the current quarter was much worse than investors had expected to see, and even though Aeropostale is taking steps to try to reinvigorate its brand and reawaken some of its growth from past years, shareholders have started to worry about whether the company has enough cash to make a true turnaround. On that score, Aeropostale's arrangement to obtain financing from hedge fund Sycamore Partners might have been more counterproductive than encouraging.

Raptor Pharmaceuticals plummeted 24% after the small biotech gave discouraging guidance for sales of its key drug Procysbi for the current fiscal year. With expectations of just $55 million to $65 million in sales for the treatment for nephropathic cystinosis, Raptor's impressive growth wasn't enough to keep investors happy, especially given that the stock has soared on hopes that Procysbi would become a blockbuster drug for the company. With some other potential indications for Procysbi in the company's pipeline, Raptor could pose an interesting value for those who believe that the prospects for its key drug are brighter than shareholders seem to think after today's plunge.

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