California Code, Government Code - GOV § 9371

(a) This article applies to members who elect to be subject to its provisions and does not apply to any member while his or her services are included in the federal social security system.

(b) An election pursuant to this section shall be in writing and shall be effective only when received in the office of the board.  The board may, on the request of any member who elects to be subject to this article, cancel the election if it finds that the election was made because of the member's mistake or misunderstandings and that the member has acted diligently in making the request.  The cancellation is effective as of the first day of the month following the board's action canceling the election.