While present in almost all categories, their burden and distribution vary, with certain defects acting as clear ‘hotspots’, such as IPEX syndrome, APECED, LRBA, RAG1, RAG2, CD3γ, Helios, ARPC1B, and CD55 deficiencies. This evidence concerns the gene ARPC1B and immune dysregulation-polyendocrinopathy-enteropathy-X-linked syndrome.