Embryonic stem cells have a lot of potential in terms of treatment because they have an unlimited capacity to grow and differentiate (they are totipotent and pluripotent). Adult stem cells, or somatic stem cells, are limited in their ability to differentiate. However, there are two main problems with embryonic stem cells: 1) they are controversial in origin, as to be harvested they need to come from an embryo of 70-100 cells, taking away what could have potentially grown into a baby, and 2) embryonic stem cells want to grow into a fetus, and scientists still do not know how to control their growth potential, so as a result they can cause tumors when injected into another person.