Reversible telomere length maintenance via the hTERT binary vector. Southern blot analysis of telomere DNA fragments in the same samples shown in Figure 5. Fifteen micrograms of digested genomic DNA was loaded in each lane, and the blot was probed with a radiolabelled telomeric oligonucleotide (see Materials and Methods). The positions of the DNA markers are indicated at left, in kilobase pairs (kbp). Note the gradual elongation of average telomere length in the hTERT-OFF population switched to the 'ON' state (lanes 2–6) relative to HA-5 cells containing no vector (lanes 9–11), or hTERT-OFF cells without addition of Cre (lanes 1, 8).