FLCN and cystic kidney disease: Whereas normal tumor suppressor activity was shown in vitro,32 transgenic expression of FLCN Lys508Arg in heterozygous Flcn knockout mice was associated with bilateral cystic kidney disease, suggesting a dominant-negative effect.33 Analogously, we did not find any signs of malignancies and only minimal cystic lung manifestation in our patient, but severe bilateral cystic kidney disease with dominant transmission (Fig. 4).