sentences:  While the Lark experiments demonstrated co-segregation, the co-segregation may have been an artifact of radiation from the tritium.Although Potten identified the cycling, label-retaining cells as adult stem cells, these cells are difficult to identify unequivocally as adult stem cells.While the engineered cells provide an elegant model for co-segregation of chromosomes, studies with these cells were done "in vitro" with engineered cells.