To begin understanding the nature of this interaction, we identified the APC-binding domain of pUL21a. Sequence alignment of pUL21a with its homologues in chimpanzee CMV (CCMV) and Rhesus CMV (RhCMV) revealed a highly conserved N-terminus (residues 1-47), divergent middle region (residues 48-83), and C-terminus that contained several conserved residues (residues 84-123), including a proline-arginine (PR) pair at residues 109-110 ( Figure 2A ). We created a series of truncation mutations targeting each region in the GFP-tagged pUL21a, and tested the ability of mutant UL21a proteins to interact with the APC in 293T cells ( Figure 2B ). All mutants were expressed at similar levels and were efficiently immunoprecipitated by the GFP antibody ( Figure 2C , and data not shown). As expected, full-length gfpUL21a wt co-immunoprecipitated both APC3 and APC8 while the gfpUL21a stop mutant did not. Importantly, while the carboxylterminal fragment of pUL21a consistently co-immunoprecipitated APC3 and APC8, the amino-terminal and middle fragments were unable to do so. Thus the carboxyl-terminus of pUL21a contains the APC binding domain.