(A) Schematic diagram of the gRNA1 targeting exon 4 of the APOE gene: the four exons of the APOE gene are shown, with the gRNA1 targeting a specific region of exon 4. The gRNA contains the protospacer adjacent motif (PAM) sequence linked to the Cas9 nuclease that will induce a double strand break (DSB) 3 bp upstream of the PAM, represented with red triangles. The asterisk (*) indicates the nucleotide that determines the APOE ε3/3 background. (B) The all-in-one lentivirus CRISPR vector contains the specific gRNA under the U6 promoter and the puromycin resistance cassette (PuroR), Cas9 nuclease and green fluorescent protein (GFP) tag under the translation elongation factor 1 α (tEF1a) promoter. (C) Sequencing electropherograms of SK-N-SH cells targeting the region of interest of the APOE gene showing WT unedited cells, a double peak electropherogram from the predicted DSB region in CRISPR/Cas9 edited cells and a normal sequence in the APOE region in CRISPR/Cas9-edited cells. Asterisk indicates the nucleotide that determines the APOE ε3/3 background and red triangle indicates the predicted cut site of the Cas9 nuclease.