RRM1 and retinoblastoma: This is strengthened by the observation that the mRNAs for E2F1 and E2F2, cyclins A1, A2 and cyclins E1 and E2 as well as the mRNAs for the ribonucleotide synthase subunits RRM1 and RRM2 are high in retinoblastoma primary tumors (Supplementary Fig. 1).