GPT and acute promyelocytic leukemia: ALT cells are characterized by several hallmarks at telomeres: specialized ALT-associated promyelocytic leukemia foci (APBs), heterogeneous telomere length, abundant extrachromosomal telomeric sequences (ECTSs) such as T- and C-circles, and high levels of telomere sister chromatid exchange (T-SCE) [12,58,59].