Ex vivo gene marking of normal and malignant hemopoietic cells allows the cells to be subsequently tracked in vivo. Marking has shown that even when marrow is in remission, it may contain malignant cells that contribute to relapse. These studies have also shown that it is possible to obtain long-term gene expression in human long-lived hemopoietic progenitor cells and T lymphocytes in vivo. Current marker studies use two distinguishable vectors to track two distinctively treated cell populations in a single individual. This modification greatly increases the power of the technique. It is now possible to study the effects of purging on residual malignant cells in marrow, to determine the action of growth-promoting agents (such as cytokines and stroma) on short- and long-term repopulation by transduced marrow, and to discover which phenotypic subsets of hemopoietic progenitor cells have long-term repopulating potential. The information gained will be invaluable for improving therapeutic gene transfer protocols in which marrow-derived cells are the targets.