The multisubunit TRAP/Mediator complex is a mammalian counterpart of the yeast Mediator that shows diverse coactivation functions. Genetic ablation of the murine TRAP100 component of this complex has revealed that it is not essential for cell viability per se. However, null mutant mice die at an early developmental stage with severe malformations, and cultured TRAP100-deficient cells exhibit attenuated functions of a wide variety of transcriptional activators on ectopic reporters. The TRAP100-deficient TRAP/Mediator complex also lacks TRAP95 and TRAP150 beta/SUR2, which together with TRAP100 may form a submodule, and contains a reduced amount of SRB10/CDK8. Nevertheless, the residual complex shows unaltered binding both to RNA polymerase II and, with the exception of the oncoprotein E1A, to various activators. These findings suggest that TRAP/Mediator is broadly involved in transcription and that a TRAP100-containing submodule plays a secondary role, beyond primary activator interactions and RNA polymerase recruitment by the TRAP complex, in magnifying effects of activators on the general transcriptional machinery.