Immune reconstitution after gene therapy. (A-F) Absolute numbers of peripheral-blood immune-cell subsets, as determined by means of standard flow cytometry, over time after cell infusion. Dotted lines indicate values for T-cell counts that are defined in the protocol as representing clinically significant reconstitution at 52 weeks after gene therapy. (G) Quantity of DNA T-cell–receptor excision circles (TRECs) in peripheral-blood mononuclear cells, with a dotted line indicating the lower limit of the normal range. The values above the hash marks on the y-axis range from 201 to 40,000. From The New England Journal of Medicine, Mamcarz E et al, Lentiviral Gene Therapy Combined with Low-Dose Busulfan in Infants with SCID-X1, Volume 380, Page 1531. Copyright © 2019 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.