Main clinical features
Most individuals with SMARCB1-associated Intellectual Developmental Disorder have been diagnosed with Coffin-Siris Syndrome (CSS). These usually have hypotonia (floppy baby), feeding difficulties, delay in their developmental milestones and learning disability, distinctive facial features, sparse hair, and shortened fingers and nails (mostly the 5th finger and/or 5th toe). Some individuals may have heart and kidney problems. Features can vary and not all individuals share all problems.

Some individuals with SMARCB1 mutations do not have the same facial features seen in CSS, but have delay in their developmental milestones and learning disability with hydrocephalus (build-up of fluid in the brain).

SMARCB1 Intellectual Developmental Disorders are rare. To date, only few patients have been identified – just over 30 are known to the wider medical and scientific community. At this time, we cannot estimate how many people have this syndrome. However, it is expected that more patients will be identified thanks to the newer genetic technologies currently available.

The disorder is inherited in an autosomal dominant manner; this means that the affected persons have a change in one of the two copies of the gene, which is sufficient to cause their developmental problems. To date, all affected individuals result from a de novo SMARCB1 change, i.e. the change happened in them for the first time and was not inherited from either of the parents. The recurrence risk for future pregnancies is low (probably <1%), but greater than that of the general population because of the possibility of germline mosaicism in one of the parents (where some of the maternal egg or paternal sperm cells contain the genetic alteration or chromosome problem). Prenatal testing is theoretically feasible once the pathogenic variant is known.