Optimization steps to increase transgene knock-in (KI) efficiency in T cells: (a) Three different dsDNA template concentrations were evaluated for the best knock-in efficiency and cell viability (n = 3 for 1 and 2 µg, n = 2 for 3 µg; two-tailed t-test; ns—not significant). (b) Two different numbers of T cells were tested for electroporation (n = 5, two-tailed paired t-test, * p = 0.041). (c) Four different lengths of homology arms flanking the transgene of interest were evaluated (n = 2–4, one-way ANOVA, ns—not significant). (d) Knock-in efficiency was tested at early (4–6 days) and late (>9 days) time points post-electroporation (n = 15, two-tailed paired t-test; ** p = 0.0015).