In 2005, Korean scientists ignited one of the biggest scientific scandals in recent history when they claimed to have created the very first lines of cloned human stem cells. New analysis has confirmed that the duplicitous researchers did actually achieve a scientific milestone, producing what was likely the first line of stem cells generated from an unfertilized human egg. The process, also known as parthenogenesis, has since been replicated by other groups.

The findings were published online on Thursday by the journal Cell Stem Cell by Harvard biologists George Daley and Kitai Kim. A press release from Children’s Hospital Boston explains the methodology.

Kim, Daley and collaborators used sophisticated genetic techniques to compare mouse embryonic stem cells derived from different sources: from embryos produced by natural fertilization; from embryos produced by parthenogenesis (through artificial activation of unfertilized eggs); and from embryos created through somatic cell nuclear transfer (replacing the nucleus of an egg with the nucleus from a cell in the body). They also tested three human embryonic stem cells isolated from fertilized embryos as well as the Korean line of human cells claimed to have been created through nuclear transfer.

They discovered that parthenogenetic embryonic stem cells have a distinct genetic signature that reflects their biological origins. All cells typically contain paired sets of chromosomes, one inherited from the mother and the other from the father. During the process of parthenogenesis, one set of chromosomes is duplicated, resulting in both chromosomes of the pair being of one parental type or the other (a pattern called homozygosity, which has reduced genetic diversity). Kim and Daley showed previously that because chromosomes often exchange genetic material early in the process of cell division that creates the egg (meiosis), the duplicated chromosomes are not actually identical, but have places where the genes differ between members of the pair (called heterozygosity). In embryonic stem cells of parthenogenetic origin, this occurs especially toward the ends of the chromosomes, which are more likely to exchange genetic material, rather than the middle. In contrast, embryonic stem cells created through nuclear transfer show a consistent pattern of variation through all regions of the chromosome–thus making them easily distinguishable from parthenogenetic cells.