Rat-1 fibroblasts were transfected with either wild-type human CD40 (Rat-1wCD40) or mutant human CD40 (Rat-1mCD40). Stable transfectants were stimulated with CD40L for 72 hours. Cell lysates were prepared and equal amounts of total cellular protein was resolved on 10% SDS-PAGE. The presence of CD40 homodimers on immunoblots was determined using anti-CD40 MoAb G28.5. The numbers to the right of each panel set indicate the positions of molecular weight markers.