SLC22A1 and type 2 diabetes mellitus: Reduced functional variants of OCT1 (rs72552763/M420del, rs12208357/R61C, rs34059508/G465R, and rs34130495/G401S) were also associated with trough concentrations of metformin and improvement in HbA1c after six months of metformin treatment in a cohort of 151 T2D patients in a prospective multicentre South Danish Diabetes Study 53.