Last week, Federal Reserve Chief Janet Yellen dealt a blow to the biotechnology sector when, in a rare move for any Fed chair, she said valuations for smaller biotech firms “appear substantially stretched.”

While arguably inappropriate for any Fed lead to specifically isolate sectors, Yellen’s commentary sent biotech stocks and exchange traded funds tumbling. The iShares Nasdaq Biotechnology ETF (NasdaqGM: IBB), the largest biotech ETF by assets, is off 3.3% over the past five trading sessions.

Yelllen’s biotech bashing was particularly bad for the SPDR S&P Biotech ETF (NYSEArca: XBI), which as an equal-weight fund, features a healthy allocation to small- and mid-cap biotechs. XBI is off almost 6.6% over the past week. [Dealing With Health Care Valuations…Again]

Biotech ETFs, IBB in particular, will get a chance to have their say in the matter this week amid an earnings deluge. Starting Tuesday, 14 members of the Nasdaq Biotechnology Index, IBB’s underlying index, deliver earnings updates. Over 40% of the Nasdaq Biotechnology Index’s weight steps into the earnings confessional this week, according to data from NASDAQ OMX Global Indexes.

On Wednesday, three of IBB’s top-10 holdings – Gilead Sciences (NasdaqGS: GILD), Biogen Idec (NasdaqGS: BIIB) and Illumina (NasdaqGS: ILMN) – deliver earnings updates. Those stocks combine for almost 20% of IBB’s weight.

Two more IBB top-10 holdings step into the earnings confessional on Thursday, including Celgene (NasdaqGS: CELG), the ETF’s largest holding. The other is Alexion Pharmaceuticals (NasdaqGS: ALXN). Celgene and Alexion combine for almost 15.2% of IBB’s weight.

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