IT IS only half a cup of blood, but it could change your life. Blood taken from a newborn baby's umbilical cord is a rich source of uniquely potent stem cells. Parents are often encouraged to donate it to a public bank, so that it might be used to treat others with rare blood disorders. However, there is no guarantee you will get to use your own blood later if you need it. This was not a problem when the disorders it treated were exceptionally rare. But this is changing.
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