(A) Patient (Pt) pedigrees showing recessive inheritance of disease and HEM1 amino acid substitutions. Red symbols: deceased affected siblings, unknown genotype; N/D: not determined. (B) Chest CT scans showing ground glass opacity and pneumonia (red outline) in Pt 1.1 (upper left), bronchiectasis (red arrow) in Pt. 2.2 (bottom left). Key shared clinical features (right). (C) Structural location of patient variants in HEM1 in the WRC (PDB 3P8C, PMID 21107423). HRS: HEM1 regulatory site. (D) Immunoblot of WRC components in lysates derived from Pt and normal control (NC) CD4+ (left) and CD8+ (right) T cell blasts. (E) Quantification of WAVE2 co-immunoprecipitated by WT or mutant HEM1-Flag constructs in six independent experiments. Statistical analysis was performed using a one-sample t-test. (F) Pyrene-actin polymerization assay with WRC230VCA containing HEM2 WT or M373V, with or without activation by a Rac1-Arf1 heterodimer pre-loaded with GMPPNP. Inset: Coomassie blue-stained gel showing GST-Arf1 pull-down of WRC230VCA containing HEM2 WT or M373V and Rac1 (Q61L/P29S). Data are representative of four independent experiments. [**P ≤ 0.01, ***P ≤ 0.001.]