M45 is a 97-kDa virion-associated protein encoded by mouse cytomegalovirus (MCMV) (Krause et al., 2014, Journal of Virology, 88(17), 9963–9975; Rawlinson et al., 1996, Journal of Virology, 70(10), 8833–8849). Likewise, its HCMV counterpart, UL45 (RRL), M45 is a putative homolog of the large subunit R1 of cellular ribonucleotide reductase (RNR), an enzyme that converts ribonucleoside diphosphates into the corresponding deoxyribonucleoside diphosphates, required for de novo synthesis of both viral and cellular DNA (Lembo et al., 2000, Journal of Virology, 74(24), 11557–11565). MCMV infection is known to trigger cellular necroptosis as a part of antiviral response, but M45 acts as a cell death suppressor by binding to receptor-interacting protein 3 (RIP3) via its RIP homotypic interaction motif (RHIM) (Daley-Bauer et al., 2017, http://doi.org/10.1073/pnas.1616829114). In addition, early after infection, M45 contributes to transient activation of NF-κB signalling, but later on it blocks NF-κB-activating pathways by interacting with RIP1, NF-κB essential modulator (NEMO) and DNA-dependent activator of IFN-regulatory factors (DAI). (Krause et al., 2014, Journal of Virology, 88(17). Our anti-M45 antibody M45.01 works in IF (on MCMV infected cells), intracellular FACS (on MCMV infected cells) and each LOT is validated for Western blot (of MCMV infected cell lysates).