In recent years, stem cell research has become an important field in the developmental biology because of the greatest medical application for regenerative medicine. In this class, I like to discuss about adult stem cells using the small intestine as a model system. What is the definition of adult stem cells? How was the stem cell discovered and proved to be right or not? I will introduce two papers. The first paper clearly showed that Lgr5-positive cells act as stem cells. However, the second paper also clearly showed that the intestine is normally maintained even in the absence of Lgr5-positive stem cells. In fact, stem cells might be highly plastic than we thought.