In eukaryotes the transcriptional control of RNA polymerase II-mediated gene expression is exerted by cis-acting regulatory DNA elements classified as promoter and enhancer sequences. These elements are composed of a number of different protein binding sites. The regulatory factors that recognize such 'modules' may be ubiquitous, tissue- or stage-specific, and positively or negatively acting. According to this model the transcriptional activity of a given gene is programmed by a combination of different modules. We analysed such a site of protein-DNA interaction, the octamer motif, in the enhancers of the simian virus (SV40) early genes and the murine immunoglobulin heavy-chain gene, and in the distal sequence element (DSE) of the U2 small nuclear (sn)RNA gene of Xenopus laevis. The corresponding DNA-binding factor appears to be the same in the three cases. Moreover, a fraction containing partially purified octamer motif binding factor has a stimulatory effect on transcription in an in vitro system.