FINRA Rule 4210 sets forth the margin requirements that determine the amount of collateral investors are required to maintain in their margin accounts for a wide range of securities transactions. The SEC recently approved amendments to that rule to establish margin requirements for To Be Announced transactions (including adjustable rate mortgage transactions), Specified Pool Transactions and certain forward-settling U.S. government agency mortgage-backed securities transactions. The change is important for hedge fund managers trading in those securities both because of the new requirements for posting initial and variation margin and because managers will have to enter into new agreements (or modify existing agreements) with their FINRA-member broker-dealers to govern such transactions. This article highlights the new requirements that are relevant to private fund managers. For a discussion of margin requirements on swaps and derivatives, see “Hedge Funds Face Increased Margin Requirements Under Final Swap Rules (Part One of Two)” (Feb. 18, 2016); and “Dechert Webinar Highlights Key Deal Points and Tactics in Negotiations Between Hedge Fund Managers and Futures Commission Merchants Regarding Cleared Derivative Agreements” (Apr. 18, 2013). For margin requirements in Europe, see “Europe Approves Hedge Fund Use of CFTC-Authorized Central Counterparties Under EMIR” (Mar. 31, 2016).