735 ILCS 5/8-1601
Whenever any deed, mortgage, conveyance, release, power of attorney or other writing of, or relating to the sale, conveyance or other disposition of real estate, or any interest therein, or any other instrument in writing not required by law to be attested by a subscribing witness, may be offered in evidence in any action pending in any court of this state, and the same appears to have been so attested, and it becomes necessary to prove the execution of such deed or other writing otherwise than as now provided by law, it shall not be necessary to prove the execution of the same by a subscribing witness to the exclusion of other evidence, but the execution of such instrument may be proved by secondary evidence without producing or accounting for the absence of the subscribing witness or witnesses.
735 ILCS 5/8-1601