Summary

Advancements in gene therapy stagnated until biologists discovered embryonic stem cells (ES cells). These unique types of cells "can give rise to other functional cell types [...] and it can renew itself" (418). ES cells can also be isolated from an embryo and grown in a lab. With ES cells, "scientists learned to make genetic changes [...] in targeted positions in the genome" (420). Furthermore, "if the gene-modified ES cells eventually produced sperm and egg cells [...] then the gene would be transmitted [...] across generations" (420). In this way, "a scientist could intentionally manipulate virtually any chosen gene and incorporate that genetic change permanently into the genome of an animal" (421). However, this was not yet possible in humans.

Instead, gene therapists concentrated on delivering genes to non-reproductive cells to treat genetic conditions in an individual, if not change that person's entire genome and cure...