The notion that oligonucleotides can modulate gene‐specific expression was established more than a decade ago. Recent advances in molecu‐lar genetics have broadened the armamentarium used to manipulate gene expression in biological systems including triplex DNA, antisense RNA/DNA, and ribozymes (catalytic RNA). These oligonu‐cleotides demonstrated important early application to the elucidation of cellular signaling pathways. More recently, studies with these agents have probed their utility as potential therapeutic agents, especially in the realm of cancer. With the implementation of gene therapy in early clinical trials, oligonucleotide‐ mediated suppression of gene expression has emerged as an important strategy for gene therapy. This review will discuss the current knowledge in this field, focusing on the biology of triplex DNA, antisense oligonucleotides, and ribozymes.—Scanlon, K. J., Ohta, Y., Ishida, H., Kijima, H., Ohkawa, T., Kaminski, A., Tsai, J., Horng, G., Kashani‐Sabet, M. Oligonucleotide‐mediated modulation of mammalian gene expression. FASEB J. 9, 1288‐1296 (1995)