To test the specificity and validity of the Cy3‐labeled PNA probe, and to develop a series of positive controls for telomere length assessment in paraffin‐embedded tissue sections, we first evaluated telomere length in seven cell lines. Each cell line was fixed in formalin and embedded in paraffin to imitate standard slides of patients' tumor tissue. Representative images of the hybridization reaction localizing the fluorescent telomeric PNA probe in each cell line are shown in Figure 1A–C. Procedures for measuring telomere length by ImageJ are shown in the supplementary material, Figure S1A, B, as previously described 18. The highly metastatic cell lines (HCC‐LM3, MHCC‐97H, and MHCC‐97 L) (Figure 1A) showed minimal or no intensities of telomere FISH, while cells with low metastatic capability (Huh7 and SMMC‐7721) (Figure 1B) and the normal liver cell line L‐02 (Figure 1C) showed strong intensities, indicating the involvement of telomere length in HCC aggressiveness. In addition, Hela cells showed a high telomere signal (Figure 1C). Quantitative analysis of telomere length in cell lines is summarized in Figure 1D.