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[4] J. Kloek, G.M.J. Beijersbergen van Henegouwen, Prodrugs of 5-aminolevulinic for photodynamic therapy, Photochem. Photobiol 64 (1996) 994. J.M. Gaullier, K. Berg, Q. Peng, H. Anholt, P.K. Selbo, L.W. Ma, J. Moan, Use of 5-aminolevulinic esters to improve photodynamic therapy on cells in culture, Cancer Res. 57 (1997) 1481.