Embryonic and extraembryonic stem cell lines derived from single mouse blastomeres

Politically correct stem cells The usual methods for establishing human embryonic stem cell lines are controversial as embryos are destroyed in the process. Two papers in this issue present different means of deriving such cells from mice without the loss of a cloned embryo with potential to develop. Meissner and Jaenisch describe a proof-of-principle experiment in mice based on altered nuclear transfer (ANT), a process that creates blastocysts unable to implant in the uterus. The new technique involves blocking the Cdx2 gene, but as Cdx2 might be needed for therapeutic strategies using these cells, the gene knockout is made reversible and Cdx2 can be turned back on once an embryonic stem-cell culture is established. ANT is one alternative being discussed in US Congress hearings on the possible use of NIH funds for stem-cell research (tinyurl.com/dhwvx). Chung et al. describe a new way of deriving mouse embryonic stem cell lines that does not disrupt the embryo's ability to implant into the womb and develop. This procedure is based on pre-implantation genetic diagnosis (PGD) as used in fertility treatments, where embryos are screened for genetic defects. If healthy, they are then implanted into the uterus and develop normally. If the procedure can be transferred to human embryos already undergoing PGD, it may be possible to create banks of human embryonic stem cell lines without additional risk to the embryos.