We validated these interactions in HCMV infected cells by coimmunoprecipitation followed by immunoblot analysis. Here we used APC3 and APC8 as the marker for the APC complex. Pulldown of pGFP-UL21a, but not the GFP control, isolated both APC3 and APC8 ( Figure 1B ). The lower band detected by APC8 antibody was nonspecific as it neither co-immunoprecipitated with APC3 antibody ( Figure 1B ) nor was affected by shRNA knockdown of APC8 ( Figure S1 ). In the reciprocal experiment, APC3 antibody co-immunoprecipitated APC8 and pGFP-UL21a but not GFP. Neither GFP nor APC3 antibody co-immunoprecipitated cellular PCNA ( Figure 1B) , and an antibody against HA did not co-immunoprecipitate any of the proteins detected here (data not shown), thus providing additional evidence for the specificity of these interactions. As pGFP-UL21a is co-immunoprecipitated with multiple APC subunits, we interpret the result to suggest that pUL21a binds to the APC complex, even though the precise subunit where pUL21a directly interacts with remains unknown.