Abstract. Cervin C, Axler O, Holmkvist J, Almgren P, Rantala E, Tuomi T, Groop L, Dahlb?ck B, Karlsson E (Lund University, Malm?, Sweden, Steno Diabetes Center, Gentofte, Denmark, University of Helsinki; and Folkh?lsan Research Centre, Helsinki, Finland). An investigation of serum concentration of apoM as a potential MODY3 marker using a novel ELISA. J Intern Med 2009; doi: 10.1111/j.1365-2796.2009.02145.x.Objective. To investigate the fitness of serum apolipoprotein M (apoM) concentration as a marker for maturity-onset diabetes of the young 3 (MODY3). Study design and subjects. This study consisted of two parts. A family study included 71 carriers of the P291fsinsC mutation in hepatocyte nuclear factor-1alpha (HNF-1alpha) from the Finnish Botnia study, 53 of whom were diabetic, and 75 matched family controls. A second, case-control study included 24 MODY3 patients, 17 healthy MODY3 mutation carriers, 11 MODY1 patients, 18 type 2 diabetes patients and 19 healthy control individuals. Subjects in the case-control study were recruited from the Botnia study or the Clinic of Endocrinology, Malm? University Hospital. Serum apoM levels were measured using a novel ELISA based on two monoclonal apoM antibodies. Results. In the family study, mean serum apoM was 10% lower in female carriers of the P291fsinsC mutation compared to the family controls (P = 0.0058), a difference which remained significant after adjustment for diabetes status. There was no observed difference between groups for men. In the case-control study, no significant difference in apoM concentration was observed between MODY3 and type 2 diabetes patients, neither before nor after adjustment for total cholesterol. Conclusions. Female carriers of the P291fsinsC mutation in HNF-1alpha displayed slightly lower apoM serum levels. This difference is too small for apoM to be reliably employed as a biomarker for HNF-1alpha mutation status.