NM_001369369.1(FOXN1):c.1049C>T (p.Pro350Leu) is a missense variant predicted to cause a proline to leucine substitution at amino acid residue 350. The variant has been found, by whole exome sequencing, in at least one compound heterozygous patient with FOXN1 deficiency (P9, PMID: 33464451). The patient displayed alopecia and low t-cell numbers (less than 1,000,000 cells/L) (PP4). They also had poor proliferative response to phytohemagglutinin on total cells, though normal when evaluated on separated T cells, flow cytometric analysis showed near complete absence of CD45RA+ CD4 T cells, and they had very low TRECs. They received a thymus transplant but T cell levels were not reported post transplant. Interestingly the patients alopecia spontaneously reversed at age 3. The patient (P9 of PMID: 33464451) is compound heterozygous with pathogenic variant NM_001369369.1(FOXN1):c.246C>A (p.Cys82Ter), confirmation of trans phase not reported (PM3_supporting). The variant has a gnomADv4.1 Grpmax filtering allele frequency of 0.000003590, which is based upon the European (non-Finnish) population (9/1180006 alleles), which is lower than the ClinGen SCID VCEP threshold (≤0.00002412) for PM2_Supporting. The computational predictor REVEL gives a score of 0.893, which is above the threshold of ≥0.644, evidence that correlates with impact to FOXN1 function (PP3). Additionally, this variant is found within the DNA binding forkhead domain, a well established functional domain (PM1). In vitro assays demonstrated conflicting results, the p.Pro350Leu variant showed significantly reduced activity in a luciferase assay (<50%) from PMID: 33464451, though the variant showed only partial loss of function (88%) in another luciferase assay as part of a validated assay with pathogenic and benign controls (PMID: 37419334, PS3 not met). This may be a mild hypomorphic variant, contributing to the patients phenotype in the context of a second pathogenic variant. In summary this variant meets criteria to be classified as Likely pathogenic for semidominant T-cell immunodeficiency, congenital alopecia, and nail dystrophy due to FOXN1 deficiency based on the ACMG/AMP criteria applied: PP4, PP3, PM2_supporting, PM3_supporting, PM1 as specified by the ClinGen SCID VCEP FOXN1 subgroup (Pilot, date of approval 03/01/2024).