Cornelia de Lange syndrome (CdLS) is a rare, congenital syndrome characterized by growth retardation, dysmorphic face, mental retardation and limb reduction defects. Clinical manifestations of CdLS can be extremely variable. Mutations in NIPBL, SMC1A and SMC3 genes, encoding for a regulator and two subunits of the cohesin complex, respectively, are found in 60-65% of CdLS patients. We report on a male with CdLS who is mosaic for the c.2827delA mutation in the NIPBL gene. Allele quantitation by pyrosequencing showed the presence of the mutation in about 10% and 33% of DNA samples from peripheral blood and buccal smears, respectively. The patient shows a complex phenotype: growth and psychomotor retardation are characteristic of the severe forms of CdLS, while the absence of severe limb reduction defects and major malformations are typical of the mild phenotype. He also has depigmentation areas following Blashko lines, an unusual finding in CdLS, which has been associated with mosaicism in other genetic conditions. This case represents the first evidence of somatic mosaicism in CdLS and explains the mild phenotype in the patient as compared to that predicted by a truncating mutation. Besides confirming the clinical and genetic heterogeneity of CdLS, this case also raises the likely underestimated mutation rate of known genes and points to the complexity of addressing genotype-phenotype correlations.

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