New research published in Nature Genetics could explain why some patients with type 2 diabetes mellitus (T2DM) respond better than others to the first-line therapy metformin. In a genome-wide association study of 13,123 ethnically diverse patients with T2DM, the C allele of rs8192675 in the intron of SLC2A2 (encoding glucose transporter type 2, liver; GLUT2) was associated with a 0.17% (P = 6.6 × 10−14) greater metformin-induced reduction in HbA1c levels than the T allele. The findings bring a precision medicine approach for prescribing and dosing metformin in T2DM one step closer.