ARPC1B and immune system disorder: Although hypomorphic mutations in ARPC1B, ARPC4 and ARPC5 all give rise to a DSBR defect, the clinical symptoms displayed by the affected patients are more consistent with WAS than DIAL or BCWFF syndrome i.e., they predominantly exhibit immunodeficiency and/or haematological abnormalities rather than neurodevelopmental defects (Supplementary Table 1).