» What causes Cornelia de Lange Syndrome


is responsible for producing and maintaining the core component of the cohesin complex ring. Changes in SMC1A have been found in approximately 5% of individuals with CdLS (3).

Many individuals with changes in SMC1A usually display a non-classic phenotype (3,15,16,24,29) and have fuller eyebrows, less shortening of the nasal bridge and a rounder face compared to individuals with changes in NIPBL.

40% of individuals with changes in SMC1A display a phenotype that resembles Rett Syndrome (another neurodevelopmental disorder associated with intellectual disability) more than CdLS (3,30,31).

The gene SMC1A is on the X chromosome. There are two copies of the X chromosome in all of the cells of females and only one in all of the cells of males. For the majority of genes, one of the X chromosomes in females is inactivated (turned off) to have the same balance as in males. However, some genes are not inactivated and that is the case for SMC1A (32). This means that males are typically more severely affected than females, as females have two copies of the gene, with one likely to not have a mutation (3,15). There has been a report of mosaicism for a variant in this gene in one person only (16).

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