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What: Shares of Protalix BioTherapeutics (NYSE: PLX) plunged today by as much as 20% after the company and Pfizer (NYSE: PFE) jointly announced that the European Medicines Agency, or EMA, opposed marketing of the company's drug for treating Gaucher disease.

So what: Regulators gave the drug a positive risk-benefit assessment, saying that the benefits of the medicine outweighed the risks in treating Type 1 Gaucher disease. However, the agency could not recommend marketing authorization because another company's competing drug has marketing exclusivity in the European Union for 10 years from the time of its authorization in 2010.

Now what: Protalix and Pfizer are disappointed by the decision, but the silver lining is that the agency assigned a positive risk-benefit assessment and the subsequent overall recommendation was not related to the safety or efficacy profile of the drug in question. The two companies entered into an agreement in 2009 to develop and commercialize the drug. It was approved domestically by the FDA in May.

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Fool contributor Evan Niu holds no position in any company mentioned. Click here to see his holdings and a short bio. Motley Fool newsletter services have recommended buying shares of Pfizer. The Motley Fool has a disclosure policy. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.