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Because the number of CD45+ cells from a neonatal spleen was small, we mixed spleen cells from male and female mice for STAP cell conversion. To make germline transmission more efficient, we intercrossed chimaeras in some experiments.
For the production of diploid and tetraploid chimaeras with STAP stem cells, diploid embryos were obtained from ICR strain females. Tetraploid embryos were produced by electrofusion of 2-cell embryos. STAP stem cells were dissociated into single cells and injected into day-4.5 blastocysts. In the chimaera studies with both STAP cells and STAP stem cells, we did not find tumorigenetic tendencies in their chimaeras or their offspring (up to 18 months).
