The haematopoietic system can be manipulated genetically to increase either its resistance to drugs or its sensitivity to certain agents. Gene transfer and expression of specific drug-resistance factors might protect haematopoietic function during antitumour chemotherapy, or allow enrichment of gene-modified cells in vivo. By contrast, gene transfer of a prodrug activator, to confer sensitivity to otherwise nontoxic prodrugs, might allow deletion of engrafted cells in the event of an adverse effect such as graft-versus-host disease or the induction of a neoplasm. In addition, expression of a prodrug activator in tumour-infiltrating haematopoietic cells could provide a means of specifically activating a cytotoxic agent within a tumour mass.