In order to analyze epigenetic factors involved in the regulation of DNA replication in higher eukaryotic cells, minimal systems have to be established. We have recently constructed a non-viral episomal vector system which replicates episomally in mammalian cells and is stably maintained in the cell in the absence of selection. The potential functional elements contained in this construct are an expression cassette upstream of a chromosomal S/MAR sequence and the SV40 origin of replication. In this report we describe that an active transcription upstream of the S/MAR running into this sequence is required and probably sufficient for episomal replication. We propose a model for the activation of replication in this system which may be the basis for further analysis of replication control in other systems.