iShares Core S&P Mid-Cap 

The index measures the performance of the mid-capitalization sector of the U.S. equity market, as determined by SPDJI. The fund generally will invest at least 80% of its assets in the component securities of its index and in investments that have economic characteristics that are substantially identical to the component securities of its index and may invest up to 20% of its assets in certain futures, options and swap contracts, cash and cash equivalents.