Here, we investigated human cellular responses to targeted telomeric DSBs specifically in G1 phase, utilizing the previously characterized telomere-specific endonuclease TRAS1-EN-TRF1 (EN-T) (Anzai et al., 2001; Yoshitake et al., 2010) in telomerase-positive fibroblasts and cancer cells, and in telomerase-independent ALT cells. Here, GPT is linked to cancer.