A) A human embryonic stem cell at telophase, the final stage of the cell cycle. At this stage chromosomes (blue) are separated and encased in nuclear membranes in both the mother cell and daughter cell. Although proteins slated for destruction (green) are congregate in the mother cell, the garbage proteins are not associated with the mitotic apparatus (red) -- a molecular railroad that ships chromosomes to their final cellular destination.

B) Both cells that result from a division of embryonic stem cells are stem cells, even though one cell gets stuck with all the undegraded proteins (green). A marker of stem cell identity is a protein called Oct4 (red). De Robertis and colleagues think that the cell that takes all the garbage may be programmed for suicide.

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