Germ-line gene therapy is carried out, before cell division occurs, on the fertilized cell from which an embryo develops, whereas somatic cell gene therapy involves the direct insertion of healthy genes into cells carrying the source of a genetic disorder.
所謂種系治療是指針對尚未分化的胚胎細胞進行基因治療；體細胞治療則是將健康的基因直接植入遺傳病因所在的身體細胞中。