40 ILCS 5/7-139.4
Any participating employee who is an active member of the General Assembly on or after November 20, 1979 may, upon written application to the Board within 90 days of that date or of becoming an active member of the General Assembly, whichever is later, terminate his participation as an employee in this fund. Any person who has terminated his participation as an employee under this Section and has continued the employment upon which such employee status has been based may revoke that termination after it has been in effect at least one year by filing a notice of the revocation with the Board. Any such person may then establish service credit and creditable service for any period of service during which the termination of participation was in effect by paying into the fund, within 6 months after revoking the termination of participation, an amount equal to the employee contributions which would have been required during the period for which the termination was in effect together with interest thereon at 6% per annum compounded annually.
40 ILCS 5/7-139.4