ALT telomeres possess a number of peculiar traits commonly used as ALT-associated markers: (i) telomeres of very heterogeneous length at different chromosome ends; (ii) association of multiple telomeres in nuclear bodies containing promyelocytic leukaemia (PML) forming the so-called ALT-associated PML bodies (APBs); (iii) abundant extra-chromosomal telomeric DNA in the form of double-stranded telomeric circles (t-circles), partially single-stranded circles (C- and G-circles) and linear double-stranded DNA; (iv) elevated rates of telomeric sister chromatid exchanges9, 10. This evidence concerns the gene GPT and acute promyelocytic leukemia.