INS and type 1 diabetes mellitus: However, we have previously demonstrated GLP-1 receptor expression in insulin-positive and insulin-negative beta cells of individuals with type 1 diabetes (unpublished data: M. Boss, I. Kusmartseva, W. Woliner-van der Weg, L. Joosten, M. Brom, M. Béhe, C.J. Tack, O.C. Boerman, M.J.R. Janssen, M. Atkinson and M. Gotthardt) and have shown in rodents as well as in humans that radiolabelled exendin is a good biomarker for beta cell mass, demonstrating the high specificity of the tracer [15–18].