ARPC4 and Immunodeficiency: 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).