Editor—Deletion of the proximal end of 18q is uncommon. It has been associated with mild facial dysmorphism, hypotonia, ataxia, seizures, mental retardation, and behavioural abnormalities. Phenotypic variability has been noted. We describe a 4 year old boy with del(18)(q11.2q12.2), defined by G banding, comparative genomic hybridisation (CGH), and molecular genetic analysis. The typical facial dysmorphism was absent, but he did manifest hyperactivity, distractibility, and moderate mental retardation. This patient falls into the milder end of the spectrum of phenotypes associated with proximal 18q deletions. His case emphasises the value of performing cytogenetic analysis in children with mild global developmental delay and behavioural problems without dysmorphism. This report represents the first molecular characterisation of chromosomal breakpoints in a proximal 18q deletion case.

Proximal 18q deletion is uncommon, but is associated with a recognisable phenotype and pattern of behaviour.1-8Dysmorphic features include prominent forehead, short nose, midfacial recession, deep set eyes, and high arched palate. There is also an association with ataxia and a risk of seizures. Moderate to severe mental retardation in association with hyperactivity, distractibility, and aggressive behaviour is commonly described. A number of authors have suggested that proximal deletion of 18q represents a distinct clinical entity, although there is some degree of phenotypic variability in previously reported proximal chromosome 18q deletion cases.1 2 4 The phenotypic variability could be the result of subtle differences …