In this system, the expression of the FokI nuclease fused to telomere repeat factor 1 (TRF1) triggers damage exclusively within the telomeric DNA, leading to strong ALT features, such as telomere clustering, the formation of ALT-associated promyelocytic leukemia nuclear bodies (APBs), and POLD3-dependent telomere DNA synthesis in non-S cells. Here, GPT is linked to acute promyelocytic leukemia.