Expansion of gene-modified lymphocytes after in vivo injection of a foamy virus vector. (A) Schematic of the foamy viral vector used in the study. EF1α promoter drives the expression of GFP (to evaluate gene marking) and the coding region of the human common γ chain separated by a 2A self-cleaving peptide. Percentage of lymphocytes expressing (B) γC⁺ and (C) the GFP transgene in 5 study dogs that received foamy virus in vivo gene therapy, plotted as days postinjection. Lymphocytes in panels B and C were identified based on flow cytometry scatter gates. (D) Absolute lymphocyte counts in the 5 animals on study. Dotted horizontal lines indicate the upper and lower limits of the normal range. LTR, long terminal repeat.