In PLOS Genetics, researchers from Peking University and other centers in China and the US present findings from a single-cell sequencing-based analysis of chromosomal mosaicism in 35 donated human embryos. Using single-cell DNA sequencing, the team characterized copy number and other alterations in trophectoderm and inner cell mass tissues in the blastocyst samples and in matching human embryonic stem cell samples. The analyses suggested that mosaicism may vary depending on the tissue type tested, ranging from just over 33 percent in human embryonic stems cells to 60 percent in inner cell mass samples and almost 66 percent in trophectoderm samples — results that would be expected to impact preimplantation genetic test interpretation. "No significant difference was observed between the rate of [trophectoderm] and that of [inner cell mass]," the authors report. "However, the mosaic rate of the corresponding [human embryonic stem cells] was significantly lower than that of [trophectoderm] and that of [inner cell mass] cells, suggesting that the incidence of mosaicism may decline during embryonic development."