In the final experiments, we tested the consequence of pUL21amediated APC regulation on HCMV replication in fibroblasts. We first tested if the ability of pUL21a to regulate the APC would be responsible for its previously reported role in promoting viral DNA replication [30] . We compared the growth of ADpmUL21a PR-AA mutant virus (i.e. pUL21a APC-binding deficient) to that of wildtype, ADpmUL21a PH-AA (i.e. pUL21a APC-binding competent), or UL21a deletion viruses in multi-step growth curve analysis. We found that ADpmUL21a PR-AA grew indistinguishably from wildtype and ADpmUL21a PH-AA viruses in both cycling and G0synchronized fibroblasts, whereas the UL21a deletion virus had a 100-fold defect ( Figure 6A ) [29] . Furthermore, knockdown of Cdh1 and APC8 was unable to enhance UL21a-deletion virus replication (data not shown). This suggests that pUL21a has at least two independent activities. One is to facilitate viral DNA replication via an unknown mechanism and is responsible for the growth defect of UL21a deletion virus. The second activity is to regulate the APC, whose impact on virus replication is not apparent under the aforementioned experimental conditions.