The absorption of UV light by DNA may cause carcinogenic changes. This occurs because chemical reactions, susceptible to deteriorate the genetic code, are triggered by the excess energy deposited by the UV absorption. Up to now, it was not known how this excess energy was distributed among the bases of the DNA. It was only postulated that each base absorbs a photon individually. By using ultrafast laser pulses, we have now shown that this assumption is not valid (J. Amer. Chem. Soc.