The present invention provides a method of modulating the malignant properties of a cell in a human or other mammal by contacting a neoplastic cell with a growth modulating amount of an expressible nucleic acid sequence for ribonucleotide reductase R1 of the mammal. The present invention also provides and uses a growth modulating amount of the ribonucleotide reductase R1 protein or biologically active peptide to modulate the malignant properties of a cell in a human or other mammal. The method provides for a generally elevated expression of the R1 component of mammalian ribonucleotide reductase. The expressible nucleic acid sequence can be in the form of a vector for gene therapy.