Some studies have suggested that for trisomies, some genes are expressed far in excess of the expected 150% level and that this "dysregulation" is one of the mechanisms for the pathogenesis of trisomies. In an attempt to generalize this result to a monosomy, we examined mRNA isolated from lymphoblastoid cell lines derived from patients with 18q- syndrome, a deletion syndrome involving loss of the distal long arm of chromosome 18. Expression levels of ten chromosome 18 genes were compared between cell lines from eight patients with 18q- syndrome and four diploid controls. Gene expression was investigated by a quantitative reverse-transcription polymerase chain reaction (RT-PCR) method. With the exception of the transcription factor NFATC1, which shows a tendency towards gene dosage compensation (the expression pattern correlates with IgA deficiency), all of the other genes were expressed at a level proportional to their gene copy number. This was true regardless of mRNA abundance or different patterns of gene expression (ubiquitous versus tissue-specific gene expression). These results indicate that, unlike dysregulated gene expression apparent in some trisomies, this monosomic syndrome is largely due to consequences of reduced gene expression.