In a new report from Wu et al., (2015) extracted cells from the renal cysts of kidney-specific Flcn knockout mice (Chen et al., 2008) to investigate their tumourigenic potential and response to the mTOR inhibitor sirolimus. The cells were cultured in vitro to establish new cell lines negative for Flcn. In nude mice these cells formed sarcomatoid RCC tumours that showed increased mTORC activity. Treatment of the allografted mice with sirolimus resulted in slower tumour growth with a similar response seen in mice inoculated with the human BHD cell line UOK-257. This work provides greater evidence for mTOR inhibitors as BHD-RCC treatments.