Among the most common CID, the shortest mean diagnostic delay was observed in SCID (median: 2 months; range: 0–10 months), followed by the Omenn syndrome/SCID-GVHD (median: 2.5 months, range: 0–4.67 months), MHC class II deficiency (median: 7 months; range: 0-132 months), and hypomorphic SCID (median: 24 months; range: 0-96 months). This evidence concerns the gene HLA-C and Omenn syndrome.