40 ILCS 5/7-109.4
"Tier 1 regular employee" means a participant or an annuitant under this Article who first became a participant or member before January 1, 2011 under any retirement system or pension fund under this Code, other than a retirement system or pension fund established under Articles 2, 3, 4, 5, 6, or 18 or in any self-managed plan established under this Code, or the retirement plan established under Section 22-101.
"Tier 1 regular employee" includes a person who received a separation benefit but is otherwise qualified under this Section and subsequently becomes a participating employee on or after January 1, 2011.
"Tier 1 regular employee" includes a former participating employee who received a separation benefit under Section 7-167 for service earned prior to January 1, 2011 who returns to a qualifying position after January 1, 2011.
"Tier 1 regular employee" includes a participating employee who has omitted service as defined in Section 7-111.5 that includes any period prior to January 1, 2011 only if he or she establishes sufficient service credit under item (12) of subsection (a) of Section 7-139 to include service prior to January 1, 2011.
Notwithstanding anything contrary in this Section, "Tier 1 regular employee" does not include a participant or annuitant who is eligible to have his or her annuity calculated under Section 7-142.1 or a person who elected to establish alternative credits under Section 7-145.1.
40 ILCS 5/7-109.4