A method of identifying an undifferentiated human embryonic stem cell in a sample which may contain such cells, the method comprising identifying the cell or cells within the sample that express podocalyxin-like protein (PODXL) on their surface. A method of isolating an undifferentiated human embryonic stem cell from a sample containing such cells, the method comprising isolating the cell or cells within the sample that express PODXL on their surface. Typically, the methods use an antibody which binds to PODXL. Undifferentiated human embryonic stem cells isolated by the method may be useful in cell therapy. Also, in particular, compositions of cells differentiated from a human embryonic stem cell but which composition has been depleted of undifferentiated human embryonic stem cells are provided which are useful in cell therapy.