To more precisely test if pUL21a was able to regulate the APC in the absence of other HCMV proteins, we developed an inducible pUL21a expression system. We constructed a HeLa cell line stably expressing a GFP-tagged TetR (tetracycline repressor) gene. We then transduced this cell line with lentiviruses expressing pUL21a stop , pUL21a PH-AA , or pUL21a PR-AA under a CMV-TetO (tetracycline operator) promoter. pUL21a protein accumulation was only detected in the presence of tetracycline, suggesting tight regulation of pUL21a expression ( Figure 5A ), although its levels were significantly lower than those expressed in transiently transfected cells ( Figure S7A) . Importantly, the addition of tetracycline significantly reduced APC4 and APC5 protein levels in cells expressing pUL21a PH-AA , but not pUL21a stop or pUL21a PR-AA . To assess the consequence of pUL21a on APC activity, we synchronized cells expressing pUL21a PH-AA (i.e. wild-type pUL21a) in mitosis with nocodazole and then assayed their ability to progress out of mitosis after release from nocodazole treatment. In the absence of tetracycline and pUL21a, cells readily progressed through the mitotic phase following release. In the experiment shown in Figure 5B , 26% and 48% of cells entered the next G1 phase by 2 and 4 hours, respectively. In the presence of tetracycline where pUL21a was expressed, progression through the mitotic phase was clearly delayed. As the result, only 5% and 24% of cells reached G1 by 2 and 4 hours, even though by 8 hours most of pUL21a-expressing cells were able to enter G1, likely due to low expression of pUL21a in these cells as compared to those in transiently transfected cells. Additionally, following nocodazole withdrawal, APC substrates geminin and cyclin B1 remained elevated in the presence of tetracycline while their levels were reduced in its absence ( Figure 5C ). Our results provide strong evidence that pUL21a expression alone is sufficient to regulate APC activity.