All lifeforms have the Lineage Factor, and by manipulating it from a young age the growth of a lifeform can be changed. Eating a Devil Fruit affects one's lineage factor that allows them to gain a supernatural power.
By manipulating the Lineage Factor of his pregnant wife, Sora and the four fetuses, Vinsmoke Judge could make his four sons Modified humans. While arguing with his wife he claim that he'll use this procedure solely on his kids. However, due to his wife's interference, only three of their children were born modified. After their birth, they ware further experimented on and drugged, in order to increase their development, along with their elder sister.
The subjects develop an Exoskeleton (外骨格 Kaikokkaku?), lose their emotions of compassion, have an increased healing ability, and with training, rapidly increase in strength and resilience to the point equaling an adult man while in their early childhood. Reiju was also modified so that she lost some of her emotions, and is unable to refuse her father's orders. 
Vinsmoke Judge conducted several researches and experiments with the Lineage Factor, becoming able to artificially manufacture human beings to fill the soldier ranks of Germa 66. Those soldiers are known as Clone Soldiers (複製 (クローン) 兵 Kurōn Hei?) and they are made without the knowledge of their origin.
- The Lineage Factor is a reference to DNA, the molecule that contains all the genetic information of the host organism.
- ↑ 1.0 1.1 1.2 One Piece Manga — Vol. 84 Chapter 840 (p. 3-5), Yonji explains the Lineage Factor and cloning.
- ↑ 2.0 2.1 One Piece Manga and Anime — Vol. 70 Chapter 698 and Episode 625, Law explains that Caesar's research is an application of Vegapunk's genetics research.
- ↑ Reiju tell Sanji about their mother intention.