Human fibroblast growth factor 21 (FGF21) is a member of a family of the atypical fibroblast growth factors that include FGF19 and FGF23 in man. FGF21 lacks the heparin-binding domain of conventional FGFs and can consequently diffuse throughout the body and function as a hormone. FGF21 stimulates glucose uptake in adipocytes and the effects on glucose uptake is additive with insulin (1). The iLite? FGF21 Assay Ready Cells are a genetically engineered reporter gene cell line responsive to FGF21 by specific and proportional expression of Firefly Luciferase. The receptor chain FGFR1c is overexpressed on the surface together with the co-factor β-Klotho which has been genetically optimized to enhance sensitivity. Normalization of cell counts and serum matrix effects is obtained by a second reporter gene, a Renilla Luciferase reporter gene construct, under the control of a constitutive promotor.