The Human Cytomegalovirus Transmembrane Protein pUL50 Induces Loss of VCP/p97 and Is Regulated by a Small Isoform of pUL50 In this study, we demonstrated that a smaller isoform of pUL50, UL50-p26, is expressed during HCMV infection from an internal methionine codon in the UL50 gene. A mutant virus that lacked UL50-p26 expression showed a growth defect at low MOIs, and this correlated with the reduced expression of IE2 and VCP/p97