For instance, Celgene (NasdaqGS: CELG) posted lower-than-expected fourth-quarter earnings due to higher costs. The company’s net profit dipped $561 million, or 69 cents per share, in the fourth quarter from $613.9 million, or 74 per share, year-over-year.

Celgene is the second largest component in IBB’s portfolio at 9.2% of the fund’s overall weighting. XBI, on the other hand, follows a more equal-weight indexing methodology and only holds a 1.8% weight in CELG. However, XBI includes a greater tilt toward mid- and small-sized biotech companies.

iShares Nasdaq Biotechnology ETF

Max Chen contributed to this article.