A gene is provided, which can be used as a marker for determining whether a certain cell, particularly an undifferentiated cell including a tissue stem cell, has pluripotency or an undifferentiated state. The gene is called Stm and includes a Stm1 gene, which is expressed specifically in a cell under an undifferentiated state if the cell has pluripotency. A kit for determining a differentiated state of a cell is also provided. The kit comprises (a) an agent capable of reacting specifically with a Stm gene or a Stm gene product; and (b) means for determining whether or not the Stm gene is expressed in the cell.