ALT cells are characterized by an increased incidence of extrachromosomal circular telomeric DNA (C-circles and T-circles), an elevated frequency of telomeric sister chromatid exchanges (T-SCEs), the formation of ALT-associated PML (promyelocytic leukemia) bodies (APBs, which are nuclear structures that assemble specifically in ALT-positive cells and are important for telomeric DNA synthesis), and frequently harbor inactivating mutations in chromatin remodeling proteins (such as ATRX and DAXX) or DNA damage repair factors (such as SMARCAL1 and SLX4IP) [5–7]. The gene discussed is GPT; the disease is acute promyelocytic leukemia.