Abstract: Disclosed are methods and compositions for identifying, monitoring and treating premalignant and malignant conditions in a human subject. The present invention further discloses methods and compositions for determining cells undergoing apoptosis, and for increasing the efficacy of a cancer therapy. The methods involve the use of apurinic/apyrimidinic endonuclease (APE), independently, as a marker for (pre)malignant conditions and for apoptosis. Also described are polyclonal antibody preparations for use in methods for detecting APE and methods for modulating expression susceptibility of cells to apoptosis.