Fluorescence in situ hybridization to detect telomere repeats. Quantitative in situ hybridization (Q-FISH) using Cy3-labeled (CCCTAA)₃ peptide nucleic acid probes on metaphase chromosomes from a human lymphocyte (A) and an embryonic fibroblast from a late-generation telomerase RNA KO mouse (B). Note that the fluorescence intensity is very similar for both sister chromatids at individual chromosome ends but very heterogeneous between ends. The arrows in panel B point to pseudo metacentric chromosomes resulting from the fusion of 2 acrocentric mouse chromosomes. Note the lack of telomere repeats at the junction of the 2 fused chromosomes.