Fig. 15. Application of fluorescent in situ hybridization (FISH) to cytogenetic analysis. A. Interphase FISH analysis of chorionic villi. Right panel: Three orange signals indicate three copies of chromosome 21 (trisomy 21); two green signals indicate two copies of chromosome 13. Left panel: Two aqua signals indicate two copies of chromosome 18; two green signals indicate two copies of the X chromosome (XX) (AneuVysion Assay, Vysis). B. Chromosome paint DNA FISH probe applied in the case of a morphologically distinct, familial variant of chromosome 20 ( right ). A variant was observed in amniotic fluid cells and in the peripheral blood cells of the healthy father. A normal infant resulted (WCP 20, Vysis). C. FISH probe applied in suspected case of DiGeorge syndrome. DiGeorge syndrome is a complex malformation pattern characterized by congenital heart defects, hypoplastic or absent thymus and parathyroid glands and dysmorphic facies. Metaphase cell demonstrating single signal (orange) indicative of a deletion of 22q11.2 that is common in DiGeorge syndrome. A gene control probe (green) maps close to the end of chromosome 22 (22q13) shows two signals (LSI DiGeorge/VCFS Region Probe, Vysis).