Briefly, ALT is characterized by heterogeneous telomere lengths, high levels of telomeric sister chromatin exchanges (T-SCEs) resulting from telomeric crossover events, extrachromosomal C-strand-rich C-circle formation, telomeric DNA nuclear structures called ALT-associated promyelocytic leukemia (PML) nuclear bodies (APBs), and high levels of TERRA expression [15,16]. This evidence concerns the gene GPT and acute promyelocytic leukemia.