Stem cells are characterized by the fact they are undifferentiated. This presents a major obstacle to their identification, particularly for the more primitive stem cells which are normally in a quiescent state and do not express many antigens. It is, presumably, the overall combination of positive and negative antigen markers that are important in the identification of stem cells. Even though stem cells are undifferentiated, during development they are already committed to becoming one particular cell or tissue type. Therefore, an epidermal (skin) stem cell can only form keratinocytes, and an intestinal stem cell can only form intestinal cell types present in the villus.