##  [Unit Investment Trusts](/unit-investment-trusts) 

 

One type of ETF structure, UITs blend traits of open-end and close-end mutual funds by offering a diversified basket of assets open to investors, but one that’s first issued through an initial public offering. UITs also come to market with a set number of shares and an expiration date. The SPDR S&P 500 ETF Trust (SPY) is the biggest example of a UIT, and one that has had its expiration date postponed over time.