FOXN1 and Aplasia of the thymus: FOXN1 mutations can be subcategorised by zygosity. Homozygous mutations typically result in biallelic loss of function and the “nude/severe combined immunodeficiency (SCID)" phenotype characterised by complete athymia, alopecia and nail dystrophy [3]. Compound heterozygous mutations have been associated with a range of features, including SCID features without typical nail or hair changes [4].