Box 1.

Glossary.

CRISPR-Cas9	Genome editing technology that allows for targeted, fast, and cheap modification of DNA. CRISPR-Cas9 system consists of a guide RNA molecule that identifies a targeted DNA sequence and Cas9 enzyme, which cuts the identified sequence.
Embryo	An early developmental stage of an organism, which starts after fertilization and lasts 7 weeks in humans. Experiments on human embryos are allowed until 14 days after fertilization. After this time, the embryos must be destroyed.
Gametes	Reproductive cells. Female gametes are called eggs or oocytes; male gametes are called sperm. Egg and sperm unite in the process of fertilization, which initiates the development of a new organism.
Genome editing	Technologies used to introduce changes to DNA, for example, to insert or delete a DNA sequence.
Germline	Refers to the cells that may pass on their genetic material to the offspring. Gametes and embryonic cells are examples of germline cells. Changes introduced to the DNA in germline cells are heritable.
Mosaicism	Occurrence of cells with different genotypes in one organism, for example, when a given gene is modified in some cells but not in all the cells of an organism.
Off-target effects	Unintended DNA modifications that occur outside of a targeted DNA sequence in a genome-editing process.
Oocyte	A female reproductive cell.