Several examples of photochemical DNA-cleaving molecules have been described. Photocleaving amino acid (PCA1) has been shown to cut DNA via an electron transfer mechanism. The most readily oxidizable sites in the photochemical one-electron oxidation of duplex DNA are shown to be 5'-guanine of -GG- steps. An important and general rule, referred to as the 'guanine-guanine stacking rule', has been proposed for one-electron transfer from B form DNA. This rule can predict the most electron-donating sites in B form DNA and is important for understanding the HOMO-LUMO interaction of B DNA with electron-accepting molecules. (C) 1997 Elsevier Science S.A.