SK-N-SH cells and selected clones 1–4 were cultured with 10 µM ATRA or DMSO control for 6 days. (A) Schematic representation of the APOE gene showing the four exons. Black arrow shows the targeted DSB region with the lentivirus containing gRNA1. (B–E) The mRNA expression of APOE using four sets of primers targeting the regions showed with grey arrows in (A) was normalised to the housekeeper genes GAPDH and HPRT1 and SK-N-SH. The expression levels in DMSO vehicle control conditions was used as 1 for reference. Data in (B–E) are derived from one representative experiment that was performed in triplicate. Individual data points are shown. Histogram bars represent mean values and error bars represent S.E.M. Graphs show **P<0.01 and ***P<0.001, compared with each DMSO condition by two-tailed t test; ^###P<0.001 compared with the SK-N-SH DMSO condition and ^†††P<0.001 compared with the SK-N-SH ATRA condition by one-way ANOVA with Dunnett’s post-hoc analysis.