Transduction efficiency of the infused product and number of transduced cells are variable from patient to patient. The lack of selective advantage for gene-marked ADA-expressing granulocytes results in a similar proportion of transduced granulocytes to that found in hematopoietic stem cells (HSCs). In contrast, gene-marked lymphocytes expressing ADA exhibit a strong selective advantage, resulting in a higher percent transduction and vector copy number (VCN) in lymphocytes compared with granulocytes. On the left, a low transduced HSC number results in lymphocytes bearing a limited number of unique integrants and persistent lymphopenia. On the right, high transduced HSC numbers translate to better correction of lymphopenia and polyclonal repertoire with higher number of unique integrants.