Introduction
While account holders are always at risk of having a short security position closed out if IB is unable to borrow shares at settlement of the initial trade or bought in if the trade settles and the shares are recalled by the lender thereafter, certain securities have characteristics which may increase the likelihood of these events occurring. Two examples are leveraged Exchange Traded Funds (ETF) and Exchange Traded Notes (ETN), where the supply of shares available to borrow can be influenced by a number of factors not found with shares of common stock. An overview of these securities and these factors is provided below.


Overview
As background, an ETF is a security organized as a pooled investment vehicle that can offer diversified exposure or track a particular index by investing in stocks, bonds, commodities, currencies, options or a blend of assets. An ETF is similar to a mutual fund in that each share of an ETF represents an undivided interest in the underlying assets of the fund. However, unlike a mutual fund in which orders are only processed at a price determined at the end of the day, ETF shares are repriced and trade throughout the day on an exchange. To balance the supply and demand of shares and ensure that secondary market prices approximate the market value of the underlying assets, ETF issuers allow Authorized Participants (typically large broker-dealers) to create and redeem ETF shares in large blocks, typically 50,000 to 100,000 shares. While many ETFs invest solely in securities, others use debt or derivatives to track and/or magnify exposure to an index. The ProShares Ultra VIX Short-Term Futures ETF ( symbol: UVXY) is one example of a widely traded leveraged ETF.

ETNs are also securities that are repriced and trade throughout the day on an exchange and are designed to provide investors with a return that corresponds to an index. Unlike ETFs, however, ETNs are unsecured debt instruments and do not represent an interest in an underlying pool of assets. They do not pay interest like traditional debit instruments, but rather a promise to pay a specific return that typically corresponds to an index or benchmark. The Barclays iPath® S&P 500 VIX Short-Term Futures™ ETN (symbol: VXX) is one example of a widely traded ETN.

The supply of shares available to borrow in order to initiate or maintain a short sale position may be less stable for certain leveraged ETFs and ETNs, including UVXY and VXX, due to the following factors:

- Limited Authorized Participants: The number of Authorized Participants willing to issue ETFs, particularly those that invest in derivatives (e.g., futures contracts, swap agreements and forward contracts) rather than securities and seek performance equal to a multiple (i.e., 2x) or an inverse multiple (i.e., -2x) of a benchmark may be limited. Moreover, Authorized Participants have no legal obligation to create shares and may elect not to do so to minimize their exposure as a dealer. 

- No Authorized Participants: As ETN shares represent credit instruments, the supply of such shares is determined solely by the issuing financial institution and Authorized Participants are not involved with the creation or redemption of shares. The ETN issuer typically reserves the right to limit, restrict or stop selling additional shares at any time.

- Limited Holding Period: Certain leveraged ETFs and ETNs seek to match the performance of a benchmark index for a single day rather than an extended period. They are principally used by institutional investors and other traders looking to obtain short-term exposure to an asset class, hedge other investments in a portfolio or invest as a way to gain interim exposure to a particular market while gradually investing directly in that market. These factors can result in a higher rate of turnover and less stability of share inventory available to lend for short sales. 

- Margin Considerations: Shares made available for lending to short sellers often originate from brokers who maintain a lien on the shares as they’ve financed the purchase of the shares on behalf of clients via margin loans. Clients purchasing shares using borrowed funds are subject to regulatory margin requirements, compliance to which depends in part upon the value of the shares supporting the loan. As certain leveraged  ETFs/ETNs are designed to provide returns in multiples of their benchmark, the inherent volatility of these products may diminish clients’ ability to maintain the position and, in turn, the broker’s ability to lend the shares.