Model for TERC-ITS insertion. (a) TERC interaction at DNA double strand break. (b) Retrotranscription of TERC 3' domain. (c) Second strand synthesis. (d) Retrotranscription of TERC 5' domain. (e) Digestion of DNA/RNA junction and addition of canonical telomeric repeats. (f) Gap filling. Curved thin lines represent the telomerase RNA (TERC) in which the orange oval corresponds to the canonical telomeric template, the grey line to the nucleotides immediately adjacent to the 3' side of the template, the yellow line to nucleotides homologous to the last nucleotides of the break site; the light blue line represents the retrotranscribed 3' region of TERC. Straight thick lines represent DNA strands. The DNA involved in the double-strand break is in black, the yellow boxes correspond to nucleotides homologous to the region of TERC preceding the sequence retrotranscribed from the 3' end, the dark blue line represents the DNA strand retrotranscribed from the 3' end of TERC and the light blue line is the complementary strand. Red and orange ovals represent TTAGGG and CCCTAA repeats, respectively. Black and grey lines correspond to the sequence homologous to the nucleotides immediately adjacent to the telomeric template.