TERF1 and cancer: To isolate and compare the telomeric composition of ALT+ and TEL+ cancer cells, we performed streptavidin pull-downs from ~6 × 107 myc-BirA-TRF1 and myc-BirA U2OS and HeLa LT cells that were equally supplemented with 50 μM biotin for 24 hr (Figure 1C).