[Phenotypic variability: genetics and chance--deletion 22q11 and schizophrenia].

Haploinsufficiency syndromes, such as the deletion 22q11, the velo-cardio-facial or DiGeorge/Shprintzen syndrome characterised by chromosome 22q11 deletion, show marked variability in the clinical features. This variability may be due to chance fluctuations in expression of genes in the deleted segment, which tend to have more impact if only one copy of the gene is present. The apparent association between deletion 22q11 and schizophrenia indicates a role for a gene within the deletion in signal transduction in the brain.