The association of OX40/4-1BB heterodimer with TRAF proteins. HEK293 cells were transfected with HA-tagged TRAF1, 2, 3, 4, or 5 for 16 hours as indicated in the top of each panel. Cells were concomitantly transfected with OX40-HA and 4-1BB–FLAG or both as indicated on the left of each panel. One percent NP-40 cell extracts were immunoprecipitated with anti-OX40 Ab in panel A and immunoprecipitated with anti–4-1BB Ab in panels B-C. Immunoprecipitates were analyzed in a reduced condition by SDS-PAGE and Western blotting specific for HA or FLAG. In panel A, positions of TRAF proteins are indicated by open arrows.