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All human cells, says Dr. Murray L. Barr of the University of Western Ontario, contain something called sex chromatin.

The cells were then examined for the presence of sex chromatin, a substance found only in female cells.

Later research showed that the spots, now known as "Barr bodies" or sex chromatin, consisted of one X chromosome � the one that is inactivated after it has done its job of helping to determine femaleness.