There are some examples that show that the telomerase-based system is not essential for telomere maintenance in all eukaryotes: retrotransposons in Drosophila telomeres, satellite repeats in Chironomus, another insect (Rubin, 1978; Biessmann and Mason, 2003), and ALT in telomerase-negative human cancers (Hu et al., 2016; Zhang et al., 2019). This evidence concerns the gene GPT and cancer.