15 ILCS 520/10

The State Treasurer may enter into an agreement in conformity with this Act with any financial institution relating to the deposit of securities. Such agreement may authorize the holding by such financial institution of such securities in custody and safekeeping solely under the instructions of the State Treasurer either (a) in the office of such financial institution, or under the custody and safekeeping of another financial institution in this State for the depository financial institution, or (b) in a bank or a depository trust company in the United States if the securities to be deposited are held in custody and safekeeping for such financial institution.