Remarkably, using two distinct markers of striatal differentiation, DARPP‐32 (Figs 5E and EV4D) and CTIP2 (Figs 5F and EV4E) to identify SPNs, we found that pERK1/2 levels were significantly reduced in HD SPNs in comparison with an isogenic WT line. This evidence concerns the gene PPP1R1B and Huntington disease.