The balance between telomere crossover and non-crossover extension events dictates overall telomere length and the prevalence of the various ALT phenotypic biomarkers, including extrachromosomal telomeric repeats, ALT-associated promyelocytic leukaemia bodies and telomere sister-chromatid exchange (T-SCE) events. The gene discussed is GPT; the disease is acute promyelocytic leukemia.