RRM1 and small cell lung carcinoma: It has been described that ribonucleotide reductase (RNR), a rate-limiting enzyme of de novo deoxyribonucleotide triphosphate synthesis, can serve as a therapeutic target in SCLC as its large subunit (RRM1) induces DNA damage response and decreases the number of cells with S phase cell cycle arrest as well as it is required for the growth of SCLC cells [50].