California Code, Government Code - GOV § 31557.1

Officers and employees of a district as defined in subdivision (g) of Section 31468, become members on the date specified in the agreement between the regents and the board of supervisors relating to the transfer to the regents of the hospital in which they are employed or of the obligation to provide professional medical services at the hospital in which they are employed.  Notwithstanding Section 31564, if the agreement so provides, those employees shall cease to be members on the date of a referendum relating to coverage of those members under the Federal Old Age, Survivors, Disability, and Health Insurance Program in which less than a majority of those eligible employees voted in favor of that coverage.