SK-N-SH, SH-SY5Y, four APOE KD clones and four CRISPR control clones were cultured with 10 µM ATRA for 6 days. (A) Representative brightfield images at 6 days of ATRA differentiation. Scale bars = 50 µm. (B) Representative western blot showing intracellular apoE expression and GAPDH expression as loading control in SK-N-SH, APOE KD clone 2, CRISPR control clone 1 and SH-SY5Y cells cultured with 10 µM ATRA for 6 days. (C) Western blot showing extracellular apoE expression in the conditioned medium and Ponceau staining used as a loading control. (D) Quantification of intracellular and (E) extracellular apoE expression in SK-N-SH cells, four APOE KD clones, four CRISPR control clones and SH-SY5Y cells. (F) Cell confluence measured over 6 days of ATRA treatment. (G) Neurite length measured at days 0 and 3 of ATRA treatment and normalised to day 0 of each experiment. Neurite length could not be analysed at day 6 as the confluence of the samples was over 90%. Data in (D,E) are normalised to SK-N-SH. Histogram bars in (D–G) represent mean values and error bars represent S.E.M, n=4, whereby individual data points represent different clones as biological replicates. Graph shows *P<0.05, **P<0.01, ***P<0.001, compared with SK-N-SH cells by one-way ANOVA with Tukey’s post-hoc analysis in (D–F), and two-tailed t test in (G).