Bloomberg is reporting that Genzyme has rejected Sanofi-Aventis' offer for the company. Citing people familiar with the matter, the Wall Street Journal reported last Friday that the two companies were in the early stages of assessing each other's interest in the deal, including the appropriate price for Genzyme. The rare disease company has a market cap of $13 billion. Reuters reports that the company's shares rose more than 8 percent on speculation that Genzyme's rejection of the bid could mean that the company is looking for higher offers from other buyers. GlaxoSmithKline, which has its own rare disease unit, has been identified as a possible buyer.