MCMV early protein 1 (E1), encoded by murine cytomegalovirus (MCMV) M112-113 gene region localizes predominately to the nucleus of MCMV-infected cells (Ciocco-Schmitt et al., 2002, Virology, 294, 199–208.). E1 has four different isoforms and its transcription is regulated by viral major transactivator protein immediate early 3 (IE3) protein (Perez et al., 2013, Journal of Virology, 87(5), 2639–2647). It enhances the expression of other viral genes, particularly those involved in DNA replication (Reddehase & Lemmermann, 2013, Cytomegaloviruses: from molecular pathogenesis to intervention. Horizon Scientific Press). The expression pattern and localization of the E1 proteins show significant similarity to those of the human cytomegalovirus (HCMV) UL112-113 proteins, signifying that MCMV E1 serves as a pertinent model for assessing the role of this early gene region during viral infection (Ciocco-Schmitt et al., 2002, Virology, 294, 199–208). Our anti-M112-113/E1 antibody CROMA103 (described in Journal of General Virology (2005), 86, 2153-2161; Journal of Virology (2010) 84 (12) 6254-6261; Journal of General Virology (1998), 79, 867-876.) works in IF (on MCMV infected cells). Each LOT of our anti- 112-113/E1 antibodies CROMA103 and M112-113.02 is validated for Western blot (of MCMV infected cell lysates).