In this respect, mutations in ARPC1B, ARPC4 and ARPC5, which encode components of the Arp2/3 complex, have been identified as the cause of several genetic disorders, characterised by either immunodeficiency and thrombocytopenia or microcephaly and neurodevelopmental defects44–48 (Supplementary Table 1). Here, ARPC4 is linked to immune system disorder.