A host cell stably transformed or transfected by a vector including a DNA sequence encoding for mutant purine nucleoside cleavage enzymes is provided. The transformed or transfected host cell can be used in combination with a purine substrate to treat tumor cells and/or virally infected cells. A nucleotide sequence encoding mutant E. coli derived purine nucleoside phosphorylase proteins which can be used in conjunction with an appropriate substrate to produce toxins which impair abnormal cell growth is also provided. A method is detailed for the delivery of toxin by generation within target cells or by administration and delivery to the cells from without. Novel purine nucleosides are detailed that yield a cytotoxic purine upon enzymatic cleavage. A synthetic process for nucleosides is also detailed.