40 ILCS 5/18-115

"Beneficiary": A surviving spouse or children eligible for an annuity; or, if no eligible surviving spouse or children survives, the person or persons designated by the participant or annuitant in the last written designation on file with the Board; or, if no person so designated survives, or if no designation is on file, the estate of the participant or annuitant. If a special needs trust as described in Section 1396p(d)(4) of Title 42 of the United States Code, as amended from time to time, has been established for a disabled child, then the special needs trust may stand in lieu of the disabled adult child as a beneficiary for the purposes of this Article.