An isolated nucleic acid includes a sequence encoding a mutant human deoxycytidine kinase (hdCK) capable of converting prodrugs, such as a nucleoside analogue, into cytotoxic drugs. An isolated vector can include the nucleic acid and an isolated host cell can be genetically engineered with the isolated vector. The polypeptides can be obtained by a procedure using recombinant techniques. A pharmaceutical composition, which includes the isolated nucleic acid, the expression vector, the host cell, or an isolated mutant hdCK, can be used as a medicament, such as for the treatment of cancer or for the prevention of a viral infection. The polypeptides and nucleic acids can be used for the treatment of malignancies and viral infections, in methods of sensitizing cells to prodrugs, in methods of gene therapy, in methods of non-invasive nuclear imaging and in methods of inhibiting pathogenic agents in a subject.

Replication-defective lentiviral vectors are described. Using this vector, methods of directing evolution of a target polynucleotide of interest for obtaining variants of the target polynucleotide, methods to generate genetic variability by preparing a cell library, and methods to isolate and/or screen variants of a polynucleotide or variants of a protein able to impact the phenotype of a cell or to confer a desired phenotype to target cells and to identify theses polynucleotide variants or protein variants responsible for this phenotype are described.