Introduction and objectives: Omenn syndrome (OS) is a very rare type of severe combined immunodeficiencies manifested with erythroderma, eosinophilia, hepatosplenomegaly, lymphadenopathy, and elevated level of serum IgE. OS is inherited with an autosomal recessive mode of inheritance. Germline mutations in the human RAG1 gene cause OS. Materials and methods: In this study, we investigated a 2-month-old boy with cough, mild anaemia, pneumonia, immunodeficiency, repeated infection, feeding

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... lties, hepatomegaly, growth retardation, and heart failure. Parents of the proband were phenotypically normal. Results: Karyotype analysis and chromosomal microarray analysis found no chromosomal struc-tural abnormalities (46, XY) and no pathogenic copy number variations (CNVs) in the proband. Whole-exome sequencing identified a novel homozygous single nucleotide deletion (c.2662delC) in exon 2 of the RAG1 gene in the proband. Sanger sequencing confirmed that both the proband parents were carrying this variant in a heterozygous state. This variant was not identified in two elder sisters and one elder brother of the proband and in the 100 ethnically matched normal healthy individuals. This novel homozygous deletion (c.2662delC) leads to the frameshift, which finally results in the formation of the truncated protein (p.Leu888Phefs*3) V(D)J recombination-activating protein 1 with 890 amino acids compared with the wildtype V(D)J recombination-activating protein 1 of 1043 amino acids. Hence, it is a loss-of-function variant. Conclusions: Our present study expands the mutational spectrum of the RAG1 gene associated with OS. We also strongly suggested the importance of whole-exome sequencing for the genetic screening of patients with OS.