Gene correction in severe combined immunodeficiency. The desired targets for vector-mediated gene transfer include the most primitive hematopoietic cells. Evidence from clinical trials suggests that occasionally a multipotential progenitor is transduced but committed lymphoid progenitors may also be transduced and lead to gene-corrected lymphopoiesis. Once gene correction has been achieved, there is preferential amplification of gene-corrected cells in the lymphoid compartments.