Nucleoside diphosphate reductase large subunit (RRM1) is one of two distinct subunits that comprise nucleoside diphosphate reductase. This enzyme is essential for the production of deoxyribonucleotides prior to S-phase DNA synthesis in dividing cells. RRM1 may reflect carcinogenesis and tumor progression. RRM1 expression is highly correlated with ERCC and PTEN expression in non-small cell lung cancer. Rrm1-positive tumors predicted slow progression.