To confirm that the slower migrating forms indeed represent UL44 molecules conjugated to SUMO-1, lysates from HCMVinfected cells at 120 h p.i. or from mock-infected cells were immunoprecipitated with an antibody against UL44. Subsequent- ly, the immunocomplexes were analyzed by western blotting with an anti-SUMO-1 antibody. Three main bands of the expected molecular mass (,65, 80, and 95 kDa) were recognized in the anti-UL44 immunoprecipitate (Fig. 5C, left panel) . Finally, the same immunocomplexes were analyzed by western blotting with the anti-UL44 antibody to demonstrate that the SUMO-1 crossreactive proteins were indeed modified UL44 forms (Fig. 5C, right  panel) . Thus, these results clearly indicate that UL44 is covalently modified by SUMO-1 in HCMV-infected cells.