GPT and neoplasm: ALT tumours are nevertheless characterized by other features, including heterogeneous telomere length (with very short and very long sequences) (Bryan et al., 1997), and the presence of extrachromosomal telomeric DNA that forms double-stranded (t-circles) (Wang et al., 2004) and single-stranded (C- or G-circles) (Henson et al., 2009) circles.