Strict control of cellular immortalization via the hTERT binary vector. After introduction of a binary expression vector containing hTERT in the 'OFF' state, two clonal populations were selected, hTERT-OFF(a) (open symbols) and hTERT-OFF(b) (filled symbols). These distinct cell populations then underwent transient transfection with a control vector, pUC (triangles), or a vector expressing Cre-recombinase (squares). Note that only the cells in which Cre recombinase was introduced became immortalized (squares). The x-axis indicates the number of days, and the y-axis shows cumulative population doublings (PDL).