Mammalian telomeres are protected by the shelterin complex that contains the six core proteins POT 1, TPP1, TIN2, TRF1, TRF2 and RAP1.TPP1, formerly known as TINT1, PTOP, and PIP1, is a key factor that regulates telomerase recruitment and activity.In addition to this, TPP1 is required to mediate the shelterin assembly and stabilize telomere.Previous work has found that TPP1 expression was elevated in radioresistant cells and that overexpression of TPP1 led to radioresistance and telomere lengthening in telomerase-positive cells.However, the exact effects and mechanism of TPP1 on radiosensitivity are yet to be precisely defined in the ALT cells.Here we report on the phenotypes of the conditional deletion of TPP1 from the human osteosarcoma U2OS cells using ALT pathway to extend the telomeres.TPP1 deletion resulted in telomere shortening, increased apoptosis and radiation sensitivity enhancement.Together, our findings show that TPP1 plays a vital role in telomere maintenance and protection and establish an intimate relationship between TPP1, telomere and cellular response to ionizing radiation, but likely has the specific mechanism yet to be defined.