In addition to the lack of telomerase activity, several other key hallmarks of ALT-positive cell populations have been identified, including the presence of ultrabright telomeric DNA foci and ALT-associated promyelocytic leukemia (PML) bodies (APBs) in a subset of cells [9], generation of circular, extrachromosomal telomeric DNA species (c-circles) [10], as well as extreme overall telomere length heterogeneity [11]. Here, GPT is linked to acute promyelocytic leukemia.