A 20-mer antisense oligonucleotide complementary to a coding region in the mRNA of the R2 small subunit component of human ribonucleotide reductase. GTI-2040 decreases mRNA and protein levels of R2 in vitro and may inhibit tumor cell proliferation in human tumors in vivo. or A substance being studied in the treatment of cancer. It blocks the production of a protein called ribonucleotide reductase, which helps cells make DNA. This may kill cancer cells that need ribonucleotide reductase to grow. It may also make cells more sensitive to anticancer drugs. It is a type of antisense oligonucleotide, and a type of ribonucleotide reductase inhibitor.