| Domain | High-confidence finding | Suggested ontology IDs | Evidence caveat |
|---|---|---|---|
| Identity / scope | Attenuated **IL2RG** common-γ-chain disorder, variably called XCID, atypical X-SCID, or leaky X-SCID; residual γc function distinguishes it from classic, usually T−B+NK− SCID-X1. Requested identifier: **OMIM 312863**. | MONDO:0010730 **as requested** | OMIM/MONDO mapping should be manually verified because databases may merge this phenotype with SCID-X1 (OMIM 300400) or broader combined immunodeficiency. (pqac-00000002, pqac-00000003) |
| Causal gene | **IL2RG**, encoding interleukin-2 receptor subunit γ/common γ chain (CD132), is causal. | HGNC:6010; NCBI Gene:3561; ENSG00000147168 | Disease association is established, but “hypomorphic” requires phenotype and preferably functional confirmation rather than gene identity alone. (pqac-00000000, pqac-00000001) |
| Inheritance | X-linked recessive germline disease: hemizygous males are predominantly affected; heterozygous mothers may be clinically asymptomatic carriers. | HP:0001419 | Rare symptomatic females could theoretically result from skewed X-inactivation or chromosomal abnormalities, but no disease-specific frequency is established. (pqac-00000010, pqac-00000014) |
| Core immunophenotype | Variable **T-low B+ NK+ or NK-low/−** phenotype; normal total lymphocyte counts do not exclude disease. Common findings include low CD4 cells, inverted CD4:CD8 ratio, low TRECs, skewed TCR repertoire, dysgammaglobulinemia, impaired proliferation, and reduced cytokine-induced STAT phosphorylation. | HP:0005403; HP:0002850; HP:0005351; HP:0004313 | No single immunophenotype is universal; residual function, age, and somatic reversion produce marked variability. (pqac-00000001, pqac-00000007, pqac-00000011, pqac-00000013) |
| Hallmark clinical features | Recurrent sinopulmonary and opportunistic infections, chronic viral skin disease—especially warts/HPV and molluscum—bronchiectasis, diarrhea/enteropathy, candidiasis, failure to thrive, eczema or granulomatous inflammation, arthritis, and EBV-associated lymphoproliferation/lymphoma may occur. | HP:0002205; HP:0002110; HP:0032180; HP:0000960; HP:0002028; HP:0001508; HP:0000988; HP:0001369 | Frequencies derive mainly from small case series: among 29 literature cases, 97% had infections and 45% opportunistic infections; ascertainment bias is substantial. (pqac-00000001, pqac-00000005, pqac-00000011) |
| Molecular mechanism | Hypomorphic γc dysfunction partially impairs signaling by receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Reduced γc surface localization or γc–JAK3 coupling leads to deficient JAK3/STAT5 signaling, impaired thymopoiesis and T/NK-cell proliferation, and defective T–B cooperation. | GO:0038110; GO:0042100; GO:0030217; GO:0001779; CL:0000084; CL:0000623; CL:0000624; CL:0000625 | Cytokine pathways are affected unequally by different alleles; alternative/JAK3-independent residual STAT5 activation has been demonstrated for p.Arg328Ter. (pqac-00000001, pqac-00000002, pqac-00000008) |
| Key variants | Documented hypomorphic alleles include **NM_000206.3:c.172C>T, p.(Pro58Ser)**; **c.455T>C, p.(Val152Ala)**; **c.458T>C, p.(Ile153Thr)**; **c.664C>T, p.(Arg222Cys)**; and **c.982C>T, p.(Arg328Ter)**. | Sequence Ontology: SO:0001583 (missense); SO:0001587 (stop-gained) | Transcript/version and ClinVar classifications must be checked variant by variant. p.Arg222Cys has produced both atypical and classic X-SCID, illustrating imperfect genotype–phenotype correlation. (pqac-00000001, pqac-00000009, pqac-00000011, pqac-00000013, pqac-00000016) |
| Somatic reversion / modifier | Back mutation or compensatory second-site variation can selectively restore γc function in lymphoid clones, attenuating disease but causing mosaic, oligoclonal, or lineage-restricted immunity. | SO:0001777 (somatic variant); HP:0001442 (somatic mosaicism) | Reversion is not reliably protective: progressive B-cell loss, restricted TCR diversity, infection, and immune dysregulation can still occur. (pqac-00000009, pqac-00000013, pqac-00000020) |
| Diagnostics | Evaluate CBC/differential, lymphocyte subsets and naïve/memory populations, immunoglobulins and vaccine antibodies, mitogen/antigen proliferation, TRECs, TCR diversity, CD132 expression, and cytokine-induced STAT3/5/6 phosphorylation; confirm with **IL2RG** sequencing plus deletion/duplication analysis. Deep sequencing of sorted lineages can identify reversion. | HP:0031406; HP:0004313; NCIT:C171178 (next-generation sequencing) | CD132 expression may be normal despite dysfunctional signaling. WES/WGS or an IEI panel is useful when the presentation is atypical; CMA, karyotype, mtDNA, and repeat testing are not first-line absent another indication. (pqac-00000001, pqac-00000002, pqac-00000013) |
| Screening | Newborn dried-blood-spot TREC screening can identify some leaky SCID cases; abnormal results require prompt flow cytometry and molecular evaluation. Cascade testing is indicated for maternal relatives. | HP:0031406; NCIT:C15644 (genetic testing) | Residual thymopoiesis can yield TRECs above program cutoffs, so newborn screening does **not** exclude hypomorphic IL2RG disease. (pqac-00000003, pqac-00000019) |
| Management | Specialist immunology care; individualized immunoglobulin replacement, antimicrobial/Pneumocystis prophylaxis, rapid treatment of infections, avoidance of live vaccines when cellular immunity is inadequate, respiratory surveillance, and definitive consideration of allogeneic HSCT or investigational autologous IL2RG gene therapy. | NCIT:C15246 (hematopoietic stem-cell transplantation); NCIT:C16387 (gene therapy); NCIT:C15691 (immunoglobulin therapy) | Direct XCID treatment trials are lacking. HSCT and lentiviral-gene-therapy outcome estimates largely come from classic SCID-X1 and should not be assumed identical; early γ-retroviral therapy caused insertional leukemia. (pqac-00000017, pqac-00000018, pqac-00000021) |
| Prognosis | Course ranges from survival into adulthood with recurrent infections to progressive lymphocyte loss, bronchiectasis, enteropathy, multiorgan granulomatous disease, malignancy, or death. Early recognition before irreversible infection or immune dysregulation is considered favorable. | HP:0002721; HP:0002110 | No XCID-specific survival curve or life-expectancy estimate exists. Somatic reversion and apparently mild childhood disease do not ensure long-term stability. (pqac-00000005, pqac-00000020, pqac-00000021) |
| Epidemiology | Extremely rare; no reliable incidence, prevalence, carrier-frequency, founder-effect, ethnic, or geographic-distribution estimates are available. One literature review identified **39 atypical patients among 362 observed IL2RG mutations/cases**, approximately 10%. | ORDO prevalence class not established | The reported proportion is a literature-derived mutation/case series, not a population prevalence estimate, and is vulnerable to publication and classification bias. (pqac-00000003, pqac-00000008) |
| Animal models | Engineered **Il2rg/IL2RG-deficient mice and pigs** model γc-dependent lymphoid failure; IL2RG-edited pigs can show X-linked T−B+NK− SCID, thymic hypoplasia, and arrest of T-cell development, supporting transplantation, humanization, and gene-therapy studies. | NCBI Taxon:10090 (*Mus musculus*); NCBI Taxon:9823 (*Sus scrofa*) | Available models generally use null or large-disruption alleles and therefore resemble classic SCID-X1 more closely than human hypomorphic XCID; allele-specific leaky models are limited. |


*Table: Compact knowledge-base summary of attenuated hypomorphic IL2RG-associated XCID, explicitly distinguished from classic SCID-X1. Identifier uncertainty and evidence limitations are flagged to prevent overinterpretation.*