Editor—Partial loss of chromosome 18q (MIM 601808) results in characteristic clinical features including mental retardation, short stature, developmental delay, CNS defects, dysmorphic facies, and hearing loss. By phenotype mapping in 26 patients, Strathdee et al 1showed that a region critical for many of the 18q− features lies in 18q22-23. Later, they were able to refine this region to an approximate 6 Mb segment within 18q23.2 However, the clinical picture of 18q− patients is extremely variable, rendering a precise prediction of the clinical outcome impossible, even when the extent of the deletion is determined.1 3 4 Among the factors contributing to this phenotypic variability, the genetic background of affected patients, environmental factors, and possibly genomic imprinting of genes in 18q may play a role. The selective expression of the paternal or maternal allele of a gene responsible for a phenotypic feature5-7 might influence the phenotype of 18q− patients, depending on whether the mutation arose in the maternal or paternal germline. Most of the 18q− patients have a paternal deletion.8 Assuming deletions originate with the same frequency in the maternal and paternal germline, imprinting of maternal genes could explain a more severe phenotype in patients with a paternal deletion, leading to a higher detection rate.

Evidence for imprinting in 18q came from linkage studies of bipolar affective disorder (BPAD). While several reports have shown linkage to 18q,9-15 in some studies most of the linkage evidence derived from families with affected phenotypes in only the paternal lineage and from marker alleles in 18q11 and 18q21 transmitted on the paternal chromosome.9 10 13 16 Genomic imprinting might explain the uniparental linkage, if critical maternal genes are imprinted (inactivated) and thus inheritance of a BPAD gene predisposes to the illness only if it is inherited from …