Mo. Rev. Stat. § 169.035
A member may at the member's option, and under such regulations as the board of trustees may adopt, deposit additional sums in multiples of twenty-five dollars but not to exceed in total in any one year ten percent of the compensation on which contributions were collected during that year. Such deposits, together with interest thereon compounded in the same manner and at the same rate as for the member's contributions, shall be available to provide at the date of the member's retirement at net rates based on the actuarial assumptions then in use, a retirement allowance in addition to that provided pursuant to section 169.070. In the event of the death of a member before receiving a retirement allowance, the total amount of such accumulated deposits with interest shall be paid to the member's beneficiary, or to the member's estate if there is no beneficiary. In the event of the termination of membership before the member receives a retirement allowance, the member shall be paid the total amount of such accumulated deposits with interest if the member has contributed for more than five years, but without interest if the member has contributed for not more than five years. Such deposits may be withdrawn without interest by a member prior to the member's retirement when the member is not terminating the member's membership, in accordance with regulations of the board of trustees. In case a retired member who is receiving such an additional retirement allowance pursuant to this section dies without having received in additional retirement allowances an amount equal to the amount of the member's accumulated deposits at retirement, the difference shall be paid to the member's beneficiary, or to the member's estate if there is no beneficiary. The option provided by this section shall apply only to members who have elected to begin such deposits before September 1, 1996.
§ 169.035, RSMo