Cancer: The Significance of a Dark Spot

The moment a female mammal is conceived — whether mouse, cat or woman — she has two X chromosomes, one each from father's sperm and mother's ovum. The presence of the pair of Xs, which are reproduced in all of her body's cells, is what makes her female. But each X chromosome packs many other chemically coded instructions for the body; if both a woman's Xs remained active throughout life, she would be flooded with a double quota of some blood-clotting factors, enzymes and other vital proteins. Since she is not, something must happen to deactivate one of those...

Want the full story?

Subscribe Now

Subscribe
Subscribe

Get TIME the way you want it

  • One Week Digital Pass — $4.99
  • Monthly Pay-As-You-Go DIGITAL ACCESS$2.99
  • One Year ALL ACCESSJust $30!   Best Deal!
    Print Magazine + Digital Edition + Subscriber-only Content on TIME.com

Learn more about the benefits of being a TIME subscriber

If you are already a subscriber sign up — registration is free!