Somatic cell nuclei in cloning: strangers traveling in a foreign land.

The recent successes in producing cloned offspring by somatic cell nuclear transfer are nothing short of remarkable. This process requires the somatic cell chromatin to substitute functionally for both the egg and the sperm genomes, and indeed the processing of the transferred nuclei shares aspects in common with processing of both parental genomes in normal fertilized embryos. Recent studies have yielded new information about the degree to which this substitution is accomplished. Overall, it has become evident that multiple aspects of genome processing and function are aberrant, indicating that the somatic cell chromatin only infrequently manages the successful transition to a competent surrogate for gamete genomes. This review focuses on recent results revealing these limitations and how they might be overcome.