HTR1B and type 2 diabetes mellitus: CHRNA7 rs3826029, a promoter variant previously linked to bipolar disorder [43], showed a protective association with T2D, and particularly SIRD subtype, indicating a potentially distinct mechanism HTR1B rs6296, a synonymous variant reducing gene expression and previously linked to psychiatric and behavioral traits [44,45,46,47,48], was inversely associated with T2D overall, with stronger effects for SIRD and MARD, and distinguished MARD from MOD.