In contrast to cells with active telomerase, cells that use ALT are characterized by (a) intracellular heterogeneity in telomere length, ranging from very short to very long [8,10]; (b) ALT-associated promyelocytic leukemia (PML) bodies (APBs); (c) the presence of extrachromosomal telomeric repeats [9,11]; and (d) high levels of telomere sister chromatid exchanges (T-SCEs) [12]. This evidence concerns the gene GPT and acute promyelocytic leukemia.