Mergers and acquisitions activity in the biotechnology sector may have been in a slumber for much of 2013, but that changed last Friday when Onyx Pharmaceuticals (ONXX), a maker of cancer treatments, rejected a $120 per share takeover offer from biotech giant Amgen (AMGN).

That offer valued Onyx at a 38 percent premium to Friday's closing price. Apparently Onyx was right to reject it because the shares are up 51 percent today, trading above $131. There is speculation that it could take an offer of $130 to $140 a share or more to get Onyx to seriously consider a sale. Even at those lofty figures, there are a few companies that could afford to make offers for the San Francisco-based company.

That could put the spotlight on the following ETFs.

Health Care Select Sector SPDR (XLV) The Health Care Select Sector SPDR is not a pure play biotech ETF, but the fund does allocate nearly 16 percent of its weight to biotech names. And yes, Amgen is a top-10 holding, but that is not why XLV makes the list.

Large, traditional pharmaceuticals companies have rarely been shy about acquiring biotech companies to boost their product pipelines, particularly when facing patent cliffs. On Monday, Bayer HealthCare, not an XLV holding, and Onyx "announced the submission of a supplemental New Drug Application (sNDA) to the U.S. Food and Drug Administration (FDA) and an application for marketing authorization to the European Medicines Agency (EMA) for the oral multi-kinase inhibitor Nexava," according to a statement.

That could make Bayer a potential suitor for Onyx and Bayer's interest could get other blue chip pharma names in the game as well such as Pfizer (PFE). Johnson & Johnson (JNJ) and Takeda have a partnership on the myeloma treatment velcade that is a rival to Onyx's kyprolis, so there is a chance the J&J may take a look at a Onyx as well. J&J and Pfizer combine for 23.4 percent of XLV's weight.

iShares Nasdaq Biotechnology ETF (IBB) As the largest biotech ETF, the iShares Nasdaq Biotechnology ETF is not a surprise member of this list, but IBB merits consideration for a couple of reasons. First, the fund devotes about 10 percent of its combined weight to Amgen and Onyx with the former being the ETF's largest holding.

Second if Amgen is interested in Onyx, and that is obviously the case, it is hard to rule out Amgen rivals taking a look at Onyx. Celgene (CELG) and Gilead Sciences (GILD) could also make runs at Onyx.

As The Street reports, Celgene already has an existing lineup that is complementary to Onyx's and Celgene has shown in an interest in acquiring new blood cancer therapies.

Celgene and Gilead combine for 15.5 percent of IBB's weight.

One More Investors looking to play the suitors in biotech M&A should consider the Market Vectors Biotech ETF (BBH), which is the leader in terms of year-to-date performance among biotech ETFs. Gilead, Amgen and Celgene combine for 28.5 percent of BBH's weight. BBH also has a 4.5 percent weight to Onyx, one of the largest allocations among biotech ETFs.

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