Mich. Comp. Laws § 38.1017d

Sec. 17d. (1) "Tier 1" means the retirement plan available under this act to a member who first became a legislator or lieutenant governor before March 31, 1997 and who does not elect to become a qualified participant of Tier 2. (2) "Tier 2" means the retirement plan established pursuant to the internal revenue code that is available to qualified participants under sections 61 to 80.