In the manga, they are four scientists, who are members of Millefiore's Research Facility. They have experimented on themselves to impress their assistant and their object of affection, Iris. As they experimented more and more, they turned into mindless monsters that only heard and obeyed Iris.
The anime takes on a more moderate view, as the Deadly Stem Force were not formerly human, but artificial creatures. They were monsters created by the four scientists, who decided to experiment after being encouraged by Iris. The first prey of the Deadly Stem Force was these four scientists who created them, as Iris betrayed them.
When Iris whips the Deadly Stem Force with her Cloud Whip, the Propagation characteristic of the Cloud Flames multiplies and propagates the monsters' muscles and other organs in their bodies, allowing them to grow and, in the same sense, regenerate rapidly.
- Muscle Scrum (マッスル・スクラム, Massuru Sukuramu?): All four monsters combine their bodies in order to make a bigger monster that has much more effective attacks. It was used as a defensive wall by Iris, but it couldn't withstand Tsuna's completed X-Burner.