A rare cause of intellectual disability: Novel mutations of NFIX gene in two patients with clinical features of Marshall–Smith syndrome and Malan syndrome

Abstract Marshall–Smith syndrome (MSS) and Malan syndrome (MS) are both allelic disorders caused by mutations in the NFIX gene. MS is characterized by overgrowth, intellectual disability, distinctive facial features, and accelerated skeletal maturation. On the other hand, clinical features of MSS consist of advanced bone age, dysmorphic features, intellectual disability, and failure to thrive at birth. In this study, we presented the clinical and molecular findings of two different patients with MS and MSS as a rare cause of intellectual disability and reported two novel variants in the NFIX gene. NFIX gene sequencing revealed a novel heterozygous c.1287delC (p.G430Vfs*34) mutation in patient 1 whose clinical diagnosis was compatible with Marshall–Smith syndrome, and in the second patient, physical features consistent with Malan syndrome, was detected a heterozygous one nucleotide duplication, c.303dupC (pCys102LeufsTer17).