Wiskott–Aldrich syndrome (WAS) is caused by mutations in the WAS protein (WASp), which plays a central role in actin polymerization, with Dedicator of Cytokinesis 8 (DOCK8), a guanine nucleotide exchange factor, and the Actin-Related Protein 2/3 (ARP2/3) complex [509]. Here, ACTR2 is linked to Down syndrome.