A rally of more than 50% in shares Onyx Pharmaceuticals (NasdaqGS: ONXX) on Monday boosted biotech ETFs, the best-performing sector in early trading.

Onyx shares soared after the company spurned a takeover bid from Amgen (NasdaqGS: AMGN).

The company said the $120-a-share cash offer “significantly undervalued Onyx and its prospects.” However, Onyx has authorized its financial advisor to contact potential acquirers.

Onyx is a top-10 holding in Market Vectors Biotech ETF (NYSEArca: BBH) at 4.5% of the portfolio.

BBH was the largest gainer among unleveraged ETFs on Monday morning with a gain of more than 6%. Biotech has been a leading sector recently. BBH was up 28% year to date through the end of June to more than double the return of the S&P 500. [Biotech ETF Rallies as Cash Tumbles In]

ProShares Ultra Nasdaq Biotechnology (NasdaqGM: BIB), a leveraged fund, rose 9% on Monday.

Other biotech ETFs rallying early Monday included SPDR S&P Biotech (NYSEArca: XBI), iShares Biotech (NasdaqGS: IBB) and PowerShares Dynamic Biotech & Genome (NYSEArca: PBE). [Falling Correlations Good for Biotech ETFs]

Market Vectors Biotech ETF