When one of the biggest names in an industry sees their stock drop by 30%, there's bound to be some collateral damage. Most biotech ETFs are down today but the damage hasn't been quite as bad as it could be.The SPDR S&P Biotech ETF (XBI) is an equal-weighted fund that owns just over 100 names. Biogen accounts for just over 1% of assets so the impact is relatively negligible. In fact, XBI is actually up more than 1.4% right now. The biggest biotech ETF, the iShares Nasdaq Biotechnology ETF (IBB), is cap-weighted so it's getting hit a little harder. Biogen makes up about 7.6% of this fund and it's down nearly 1.5%. The other larger biotech ETFs have between 3-5% of assets in BIIB and are mixed today.I own XBI in my personal portfolio and reiterated a hold rating on it to my subscribers last week.