Radiation cards come in three different forms: Alpha Rays, Beta Rays, and Gamma Rays. You attach Radiation cards to the left of Specimen or Mutant cards. All Radiation cards have an effect on their own, but also count towards bringing a Mutant card into the game when attached to Specimens.
If a Mutant card would lose all the Radiation cards it had attached to it, that Mutant de-volves back into the Specimen it once was. Any damage or Research Point tokens placed on the Mutant carries over to the Specimen card.