Cells were treated with DMSO or 0.5 μM dTAG-13 for 18 h and subsequently arrested at mitosis with 0.1 mg/ml colcemid (Roche) for 6 h (total 24 h). Metaphase chromosome spreads were prepared as previously described (Zong et al, 2019 (link); Zong et al., 2015 (link)). First, cells were induced to swell in a prewarmed hypotonic solution containing 0.75 M KCl (Sigma) for 20 min at 37°C. Next, suspensions of single cells were fixed with a solution mixture containing methanol and glacial acetic acid at a 3:1 ratio. Thereafter, cells were further washed extensively with the same fixative solution and dropped onto slides in a humidified chamber (Thermotron Industries). To visualize metaphase chromosomes by fluorescence in situ hybridization (FISH), samples immobilized on slides were sequentially treated with pepsin (5–10 μg/ml in 0.01 N HCl, 5 min at 37°C), washed, dehydrated with ethanol, briefly heat denatured (80°C, 1 min 15 s) in a slide moat (Boekel Scientific) and incubated with a commercially available Cy3-labeled (CCCTAA)3 peptide nucleic acid probe (PNA Bio) recognizing mammalian telomere sequences. After extensive washes, DNA was counterstained with DAPI. Images were acquired at 63x magnification using the Metafer automated scanning and imaging platform (MetaSystems). Fifty individual metaphases were scored for the presence of chromosomal aberrations.