RRM1 (R1, ribonucleoside-diphosphate reductase large subunit) is one of two non-identical subunits that constitute ribonucleoside-diphosphate reductase. This enzyme is essential for the production of deoxyribonucleotides prior to DNA synthesis in the S phase of dividing cells. High levels of RRM1 are associated with shortened survival in multiple myeloma patients, and it may serve as a therapeutic target of inhibition for the disease. In immunohistochemistry of normal tissue, RRM1 has cytoplasmic positivity in hematopoietic and glandular cells in most tissues throughout the body. References: The UniProt Consortium. Nucleic Acids Res. 47: D506-515 (2019); Nucleic Acids Res. 2016 Jan 4;44(D1):D733-45, PMID:26553804; Clin Cancer Res. 2017 Sep 1;23(17):5225-5237, PMID: 28442502;