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Puzzling feature
Contamination also explains one of the most puzzling features of the original work: why the alleged STAP cells were found to be capable of forming placental tissue, when embryonic stem cells are not, according to a review paper also published on 23 September in Nature. Even as people started to believe that the purported STAP cells were just embryonic cells, there was no explanation of why the researchers would see placenta forming. But the latest analyses show that a mixture of trophoblast stem cells — cells that form the placenta in a developing embryo — were mixed with embryonic stem cells and that the mixture had been used in the experiments producing the placenta.