An exchange-traded fund that tracks the Standard and Poor's Midcap 400 Composite Price Index. The organization issuing the SPDR owns each of the stocks traded on the S&P Midcap Index in approximate ratio to their market capitalization. SPDR shares can be bought, sold, short-sold, traded on margin, and generally function as if they were stocks. Investors use midcap SPDRs (and indeed all exchange-traded funds) as a way to easily diversify their portfolios at relatively low cost. Investors also see the demand for midcap SPDRs as an indicator of which direction the market believes the S&P Midcap Index is going.