Telomere PNA‐FISH revealed that compared to Eμ‐myc lymphomas, ~35% of Eμ‐myc; Pot1b∆/∆; p21+/− chromosomes displayed telomere‐free ends and ~1.5 end‐to‐end chromosome fusions per metaphase, both indicative of increased telomere dysfunction (Fig. 4B,C). The gene discussed is CDKN1A; the disease is lymphoma.