D, DIV 3+2 cortical neurons co-transfected with FlucWT-EGFP (green) and HTT-Q97-mCherry (magenta) and treated with 1 mM 4-PBA (lower row) or vehicle control (upper row) from DIV 3. Cells were stained for MAP2 (grey) as a neuronal marker, nuclei were labeled with DAPI (blue). Arrowhead points to Fluc-EGFP foci. Corresponding cultures transfected with control HTT are shown in Fig. EV1D. E, Quantification of the fraction of double-transfected cells showing Fluc-EGFP foci. N=4 biological replicates. Two-way ANOVA with Tukey's multiple comparisons test. ANOVA: HTT, **p=0.0046; 4-PBA, n.s.; HTT x 4-PBA, n.s. Significant pairwise comparisons are indicated on the graph. D 