The MERS MIN is an 18-digit loan identifier required for loans registered in the proprietary MERS system. The primary purpose of the system is to track ownership interest in registered mortgages. The number itself may be auto-generated by software or assigned directly by MERS. A portion of the number can be either randomly generated or created from an existing loan number created by the lending institution; the string would then be wrapped by a unique MERS residential organizational ID and a check digit. Once the MIN is assigned, it does not change for the life of the loan.

Currently, Fannie Mae and Freddie Mac require all e-mortgages to be delivered registered with a MERS MIN, contributing to adoption rates among originators in the market. There are membership fees associated with registering loans on the MERS system.

But while MERS enjoys significant industry participation, many loans that never end up on the MERS System are assigned MINs. In addition, the MIN naming convention is assigned and not based entirely on loan-specific information, making it nearly impossible to recreate a loan's MIN.