BACKGROUND: IInterstitial 21q deletions can cause a wide spectrum of symptoms depending on the size and the location of the deletion. It has previously been suggested that the long arm of chromosome 21 can be divided into three regions based on the clinical severity of the patients and deletion of the region from 32.3 Mb to 37.1 Mb was more crucial than the deletion of other regions.

CASE PRESENTATION: In this study we describe a female patient with dysmorphic features, hepatomegaly, thick myocardium and psychomotor delay. Conventional karyotyping was initially interpreted as full monosomy 21, but subsequent chromosome microarray analysis suggested an approximately 18 Mb partial monosomy. Re-evaluation of the karyotype and fluorescence in situ hybridization revealed deletion of the proximal 21q11.2-q22.11 segment and insertion of 21q22.11-qter to 12qter. The deletion of the present case overlaps with two of the proposed regions including part of the proposed crucial region.

CONCLUSIONS: This report emphasizes the relevance of investigating suspected full monosomies with high resolution methods and FISH in order to investigate the extent of the deletion and the presence of more complex rearrangements.