Only 4% of Down syndrome (DS) cases have a Robertsonian translocation (ROB). The aim of this study was to define the possible breakage area in 21p where ROB occurs. We prospectively and consecutively collected ten cases ROB DS from three medical centers. Of the ten DS children, six were de novo (60%), and four were due to paternal or maternal inheritance (40%). They consisted of four der(21q;21q), four der(14q;21q), one der(13q;21q), and one der(21q;22q). The origin of the extra chromosome 21q was maternal in five of six de novo ROB and paternal in one case. All four der(21;21) ROB DS were an isochromosome. The result of gene dosage change by real-time quantitative polymerase chain reaction (PCR) was compatible with array-comparative genomic hybridization in one case. We further used real-time PCR to detect the copy number of TPTE and BAGE2 located on 21p11 and SAMSN1 on 21q11. The ratio of copy number in 21p:21q was 1:3 in der(21q;21q) but 2:3 in der(13q;21q), der(14q;21q), and der(21q;22q). Our preliminary results demonstrated the critical breakpoint of chromosome 21 involving ROB might lie between BAGE2 and the centromere, located from 10.1 to 12.3 Mb.