We describe the first case of a live-born neonate with mosaic tetrasomy 21q confirmed by fluorescence in situ hybridization (FISH). A 38-year-old Caucasian female presented for amniocentesis for maternal age. Initial chromosome analysis of the amniocytes using GTG-banding showed a mos47,XY, +?i(12p)/46,XY karyotype. Follow-up studies with FISH identified the isochromosome as an i(21q); mos47,XY, +i(21q)/46,XY. The patient was delivered at 38+ weeks gestation and umbilical cord blood samples were obtained. Chromosome analysis of 43 cord blood lymphocytes demonstrated a 46,XY karyotype in all cells. However, peripheral lymphocytes taken 1 day after birth showed 1 out of 120 lymphocytes to have an extra chromosome determined to be an i(21q). While initial clinical exam of the neonate revealed similarities to Down syndrome, long-term follow up of our patient will be required to provide the first definitive description of the mosaic tetrasomy 21 syndrome.
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