Fibroblast growth factor receptor 4 (FGFR4) gene expression is activated in late-stage beta-cell tumors that develop in transgenic mice harboring SV40 large T antigen (Tag) gene that is under the transcriptional control of the insulin promoter (RIP-Tag). The FGFR4 gene was active in cell lines derived from tumors but not in cells derived from hyperplastic islets. We used both gain-of-function and loss-of-function FGFR4 transgenic mice to determine whether FGFR4 modulates islet cell tumorigenesis and, if so, to identify the nature of the effect. Both types of FGFR4 transgenic mice were viable and fertile and developed islet tumors when crossed with RIP-Tag mice. Remarkably, there was no significant perturbation in the tumorigenesis pathway resulting from either chronic up-regulation or absence of FGFR4 gene expression. Analyses included the incidence and size of tumors, rate of cell proliferation, cell density, and life span. We conclude that FGFR4 gene activation is a marker of but is not causal for beta-cell transformation.