Experiments concerning gene transcription in Xenopus oocytes have revealed that the efficiency of the HSV-TK promoter is dependent upon the nature of the attached gene sequences. Transcriptional efficiency of the TK promoter when attached to its own gene was 30-fold higher than that observed when the promoter was attached either to an avian keratin gene or to the chicken histone H2B gene. Furthermore, attachment of the keratin gene promoter to the TK gene resulted in a 20-fold increase in keratin promoter efficiency. It was found by subsequent experiments that the TK gene is able to exert a stimulatory influence upon attached promoter sequences. This cis effect was shown to be independent of orientation but dependent upon the distance between the DNA sequences involved.