(A) Flow cytometry profiles of engineered L1 retrotransposition assays in HEK293T cells co-transfected with active and mutant human L1 constructs (99-gfp-LRE3 and 99-gfp-JM111) containing EGFP retrotransposition cassettes, and either Strep-tagged Tex19.1, APOBEC3A (positive control) or empty vectors (pBSKS for APOBEC3A, pIBA105 for Tex19.1). EGFP fluorescence is plotted on the x-axis and side scatter on the y-axis of the flow cytometry profiles, and cells classed as EGFP-positive are shown in green. 99-gfp-JM111 carries the ORF1RA mutations and is severely impaired for retrotransposition (Han and Boeke, 2004; Moran et al., 1996). **p<0.01 (t-test, p=0.0001, 0.003, 0.2, 0.7 for each pairwise comparison with vector from left to right). (B, C) Blasticidin-resistant colonies from L1 retrotransposition assays in U2OS cells. Human (JJ101/L1.3) and mouse (JJL1SM) engineered L1 constructs containing blasticidin-resistance retrotransposition cassettes were co-transfected with Strep-tagged mouse Tex19.1, Strep-tagged human TEX19, or empty vector (B), or with β-ARRESTIN (negative control), APOBEC3A (positive control) or empty vector (C). Quantification of L1 retrotransposition normalized for transfection efficiency is shown. *p<0.05; **p<0.01 (t-test, p=0.0004, 0.008, 0.005, 0.014 for each pairwise comparison with vector from left to right for B; t-test p=0.2, 0.008, 0.3, 0.002 for C).