The ability of a stem cell of a cell group to develop into a differentiated cell is part of normal developmental processes. Tumor cells have completely or partially lost this possibility. We call tumour cells that still exhibit a large part of the characteristics of a differentiated, normal cell well differentiated. If such characteristics are missing, we call the tumor cell undifferentiated or de-differentiated. The more undifferentiated a tumor cell, the more malignant its behavior.