"Valuation metrics in some sectors do appear substantially stretched—particularly those for smaller firms in the social media and biotechnology industries, despite a notable downturn in equity prices for such firms early in the year," said remarks in the full policy report accompanying Yellen's testimony.
It wasn't her words that caused the biotech drop, it was the shocking personal display and gesture she made to illustrate the word "stretched." It was unspeakable - several people present actually vomited.
With a Billion dollar drug on deck and a NDA pending this stock is going to make a major move higher in no time. Note she gives not one detail about which stocks, what % of the biotechs is over valued etc. This is a generalization and I think there are some Biotechs over valued, but ACAD is not one of them.