ALT cells are characterized by frequent telomere sequence exchanges, clustering of telomeres in promyelocytic leukemia (PML) bodies (forming ALT-associated PML bodies [APBs]), long and heterogeneous telomere lengths, and extrachromosomal telomeric repeat DNA detectable as extrachromosomal telomeric signals (ECTSs) and circular telomeric G- or C-strand DNA [1,4–11]. The gene discussed is PML; the disease is leukemia.