TKTL1 and nonpapillary renal cell carcinoma: Pull-down assays of purified recombinant TKTL1 and TKT (Fig. 3b) supported their direct interaction in vitro and affinity purified endogenous TKTL1 co-purified endogenous TKT from ccRCC lysates (Fig. 3c), indicating that TKT and TKTL1 also interact with each other in vivo.