Consequences of gene editing in human zygotes. Intended outcomes appear in red boxes; unintended outcomes appear in blue type. Gene editing performed on the unicellular zygote may, in theory, be used clinically to prevent an inherited disorder by correcting a genetic defect. The resulting gene-corrected embryo could be implanted in utero for development to a live birth with every cell containing the genome edit. In theory, germ line correction of the mutation would lead to its elimination from the human gene pool. However, gene editing in human embryos to produce a live birth is consensually prohibited. In vitro research using gene editing in human zygotes may provide valuable information about human development and human disease mechanisms, which could lead to new treatments. However, several unintended consequences may occur from gene editing of human embryos, including direct cytotoxicity to the zygote from the manipulation; off-target editing and genotoxicity which could be pathogenic or be propagated to the human gene pool and may become homozygous in subsequent generations; or genetic mosaicism if gene modification occurs at a time after the zygote divides (postzygotic), leading to heterocellular gene-modification effects. Professional illustration by Patrick Lane, ScEYEnce Studios.