Atypical Down syndrome phenotype in a girl with 21;21 translocation trisomy: We describe a girl with microcephaly, short stature, coarse face, severe growth and developmental delay, seizures, hypertonia, bilateral flexion contractures of the knees, and a de novo 2121 translocation trisomy 21 in peripheral blood lymphocytes. Fluorescence in situ hybridization (FISH) analysis confirmed the trisomy 21 translocation using whole chromosome painting probe 21 (WCP21). Chromosome analysis which was also performed on skin fibroblasts and revealed mosaicism for a translocation trisomy 21 cell line (22.3%) as well as a second cell line consisting of one normal chromosome 21 and a small ring chromosome 21 derived from the translocation 21q21q (61%) and a third line consisting of monosomy 21 (16.7%). FISH analyses by LSI21 probe for the critical (21q22.2-22.3) region of Down syndrome (DS) on interphase blood cells resulted with 30% two signals and 70% three signals, skin fibroblasts showed 84% single signal, 9% two signals and 7% three signals. The size airing chromosome 21 in skin fibroblasts was very small and probably there was a large, more proximally located deletion including chromosome 21q22 band. We consider that the atypical DS phenotype of the patient originated from the small ring chromosome 21 and the monosomy 21 in the skin fibroblasts and other tissues not available for analysis. Therefore, the clinical findings of the patient were most similar to monosomy 21 mosaicism syndrome.