A Mule Cloned from Fetal Cells by Nuclear Transfer

Abstract

Nuclear transfer was used to produce a cloned mule, a sterile hybrid equine species. Mules result from breeding a male donkey (Equus asinus, 62 chromosomes) with a female horse (Equus caballus, 64 chromosomes), producing an animal with 63 chromosomes. Equine cloning has been hampered by a lack of optimal in vitro oocyte maturation and embryo culture systems (1), and previous reports of nuclear transfer pregnancies have been less than 65 days of development in utero (2).