Telomerase, the enzyme which maintains the ends of linear chromosomes in eukaryotic cells, is found at low levels in somatic stem cells but while this is incapable of preventing the progressive erosion of telomeres occurring as a consequence of cell division, such cells show greater proliferative capacity than normal somatic cells hence examination of telomerase activity in such stem cells is of interest. Our aim in this work was to examine the relationship between expression of the reverse transcriptase component (mTert) of murine telomerase. We report here the insertion of a reporter cassette comprising a segment of the promoter sequence of murine Tert gene coupled to the coding sequence of green fluorescent protein (GFP) into murine embryonic stem (ES) cells and show that this is sufficient for mimicking the expression of mTert. We show that the expression of mTert is very closely linked to telomerase activity and that both are substantially reduced upon differentiation of ES cells into more committed lineages giving us a potential reporter system for the selection and isolation of ES cells possessing different levels of telomerase activity.