Murine cytomegalovirus immediate-early 1 (IE1, pp89, m123) is one of the first proteins expressed during productive (lytic) infection as well as during reactivation from the latency. In addition to serving as regulatory co-activator transcription of viral genes, IE1 also has multiple roles in priming the cell for efficient viral gene expression (e.g. by disrupting PML or ND10 bodies or reducing histone deacetylase activity. IE1 an IE3 proteins are translated from alternatively spliced transcripts from the same locus: IE1 contains exons 1 to 4 while IE3 contains exons 1-3 and also contains exon 5. IE1 protein is 89 kDa in size. Our anti-m123/IE1 antibodies IE1.01 and Croma101 (described in Journal of General Virology (2005), 86, 2153-2161; Journal of Virology (2009), 83 (3): 1260-1270; Microbes and Infection (2003), 13 (5) 1263-1277; Journal of General Virology (2005), 86, 2153-2161; Journal of General Virology (1998), 79, 867-876) work in IF (on MCMV infected cells), immunohistology/IHC (on paraffin sections) and each LOT is validated for Western blot (of MCMV infected cell lysates).