This guide describes the interface that states with Financial Alignment Initiative Demonstrations use to conduct Medicare eligibility verification and MMP enrollment submission to the CMS MARx systems. Version 4.0 of the Guide reflects the full CMS migration from the Social Security Number (SSN)-based Health Insurance Claim Numbers (HICNs) to Medicare Beneficiary Identifiers (MBIs). The document also includes updates to reflect Infocrossing’s upgrade to the CMS real-time Medicare eligibility check via the Application Programming Interface (API) process. These changes will be implemented on Saturday, May 1, 2021.