The analysis revealed a homozygous canonical splice site variant, c.530-2A > T, in intron 4 of the NHEJ1 gene (NM_ 024782.3) (Fig. 2F); based on the American College of Medical Genetics (ACMG) guidelines this variant was classified as likely pathogenic, leading to the diagnosis of Cernunnos deficiency. The gene discussed is NHEJ1; the disease is Cernunnos-XLF deficiency.