Abstract: The present invention provides a polynucleotide which identifies and encodes a novel human GTP binding protein gamma-3 (HGPG) and HGPG itself. The invention provides for genetically engineered expression vectors, host cells containing the vector and a method for producing HGPG. The invention also provides a method for identifying pharmaceutical compositions inhibiting the expression and activity of HGPG and for the use of such compositions for the treatment of cancer. The invention also provides diagnostic assays which utilize the polynucleotide to hybridize with the transcripts encoding HGPG or anti-HGPG antibodies which specifically bind to HGPG in normal or diseased tissues.