The past decade has seen unparalleled advances in the application of molecular genetic methods to the study of neurodevelopmental disorder, including disorders with significant learning disability. Alongside this development there has been a substantial growth in the number of studies attempting to link genomic changes (deletion, reduplication, or silencing of genes) to cognitive and behaviour outcomes: in essence to link genotype to phenotype. A main benefit of this approach is that it permits an insight into the range of strengths and difficulties that can be associated with a disorder which in turn can guide the general management of children and adults with genetic causes of learning disabilities. One such disorder that has attracted attention in recent years is Cri du chat syndrome (CDC), first described by the French paediatrician Lejeune in 1963 who coined the term ‘cri du chat’ (‘cry of the cat’). Indeed, the hallmark cat cry is still regarded as an important early clinical diagnostic feature of this syndrome in some but not all affected newborn infants. Further research in the 1960s and 70s resulted in the publication of numerous case reports and a triad of clinical features became associated with CDC: the cat-like cry, dysmorphic facies, and profound global learning disability. It is now recognized that this triad does not present in all patients. Additional clinical features were also cited as being significantly over-represented in the condition. These included increased early childhood morbidity, restrictive language skills, and severely delayed psychomotor development. This somewhat pessimistic portrayal of CDC was challenged in the 1980s by the findings obtained from population-based studies and questioned more intensely in the 1990s as advances in molecular genetics allowed greater clarification of the syndrome's genotype and more detailed cognitive and behavioural studies demonstrated wider variability within the phenotype.