The PowerShares Dynamic Biotechnology & Genome Portfolio (NYSEArca: PBE) and the SPDR S&P Biotech ETF (NYSEArca: XBI) are up 8.4% and 6.8%, respectively, Monday, making the duo the day’s two best non-leveraged ETFs.

PBE and XBI are soaring because the two ETFs feature the largest weights among biotech funds to Idenix Pharmaceuticals (NasdaqGS: IDIX), shares of which have more than tripled today after Dow component Merkck(NYSE: MRK) said it will acquire the company for $3.85 billion.

That values Idenix at $24.50 per share, nearly 3.5 times where the stock closed last Friday.

As of June 6th, Idenix was PBE’s ninth-largest holding at a weight of 3%. No stock accounts for more than 5.15% of that ETF’s weight. Idenix is the fourteenth-largest holding in XBI with a weight of 1.46%. That does not sound like much, but XBI is an equal weight ETF where none of its 85 holdings receives an allocation north of 2.6%. [More Upside Possible for Biotech ETF]

Although it is an equal weight ETF, this is not the first time XBI has benefited in significant fashion from news pertaining directly to just one biotech stock. Earlier this year, the ETF surged more than 7% in one day after shares of Intercept Pharmaceuticals (NasdaqGM: ICPT) nearly quadrupled after trials for the company’s liver disease treatment proved successful. [Intercept Lifts This Biotech ETF]

The day those trial results were released, Intercept occupied just 1.6% of XBI’s weight.

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