Senescent mice are often infertile, and the cloning success rate decreases with age, making it almost impossible to produce cloned progeny directly from such animals.

Scientists have tried to produce offspring from such "unclonable" senescent mice using nuclear transfer techniques. Donor fibroblasts were obtained from the tail tips of mice aged up to 2 years and 9 months.

Although most attempts failed to produce cloned mice by direct somatic cell nuclear transfer, but they managed to establish nuclear transfer embryonic stem (ntES) cell lines from all aged mice with an establishment rate of 10-25%, irrespective of sex or strain. Finally, cloned mice were obtained from these ntES cells by a second round of nuclear transfer.

In addition, healthy offspring was obtained from all aged donors via germline transmission of ntES cells in chimeric mice.

This technique is thus applicable to the propagation of a variety of animals, irrespective of age or fertile potential.