A 2026 Nature Communications study found that serial mammal cloning has a hard biological limit. After 20 years, 58 generations, and 30,947 nuclear transfer attempts from a single donor mouse, researchers showed that harmful mutations and structural DNA damage accumulate until cloned offspring can no longer survive. The findings provide the first direct mammalian evidence of mutational meltdown, with major implications for animal cloning, conservation, livestock breeding, and the long-term limits of asexual reproduction.