A Practice Note explaining the amendments to FINRA Rule 4210, effective May 22, 2024, requiring mark-to-market margining of forward transactions in certain types of mortgage-related debt securities. Parties to these transactions may use the Master Securities Forward Transaction Agreement (MSFTA), published by SIFMA, to govern these arrangements. This Note explains the application of FINRA Rule 4210 to these transactions, sets out important background on the evolution of Rule 4210, and provides a summary of key MSTFA terms and common negotiating points.