RRM1 and glioblastoma: Inhibition of HPRT1-mediated AICA catalysis by HPRT1 depletion or 6-MP treatment and expression of RRM1 T52A exacerbated TMZ-induced DNA damage by attenuating DNA damage repair, leading to increased cell apoptosis and sensitization of GBM to TMZ treatment in mice (Supplementary Fig. 13).