The most characterized mechanisms and alterations (Figure 1) responsible for maintaining the lengthening of telomeres in tumour cells are: (1) somatic mutations of the TERT promoter (TERTp); (2) amplification of the genes TERT and TERC (3) rearrangements of the TERT gene; (4) germline genetic variants of the TERT gene and its promoter; (5) epigenetic changes; (6) ALT; and (7) non-defined TMM (NDTMM). The gene discussed is TERC; the disease is neoplasm.