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1q21.1 microdeletion syndrome
1q21.1 microdeletion syndrome is a newly described recurrent deletion syndrome with variable clinical manifestations but without the clinical picture of thrombocytopenia - absent radius (TAR) syndrome.
It has been described in 46 patients to date.
The clinical phenotype is extremely variable; the most common but non-constant clinical findings include microcephaly, developmental delay or mild intellectual deficit, slight facial dysmorphic features and eye abnormalities. Congenital malformations are not common. Autism spectrum disorders, schizophrenia or attention deficit hyperactivity disorder have been noted occasionally.
This syndrome is caused by a recurrent 1.35Mb deletion in the distal 1q21.1 region distinct from the deletion region implicated in TAR syndrome (see this term).
This microdeletion was identified by comparative genomic hybridization (CGH) microarray and is only diagnosed by molecular cytogenetics. It cannot be identified by routine chromosome analysis.
The underlying mechanism is non-allelic homologous recombination (NAHR). Deletions appear de novo or can be inherited in an autosomal dominant manner from mildly affected or completely normal parents. This suggests that the distal 1q21.1 microdeletion has incomplete penetrance and variable expressivity.