GPNMB and Birt-Hogg-Dubé syndrome: Specifically, high levels of GPNMB expression were found in all of the renal tumors (N = 11) from BHD patients, but none of the normal kidney tissues (N = 5), suggesting that GPNMB might serve as a biomarker for BHD syndrome and a therapeutic target for treatment of BHD renal tumors (Fig. 5).