A patient with the phenotype of trisomy 21 (Down syndrome) was found to have a 
normal karyotype in blood lymphocytes and fibroblasts. Assessment of the 
chromosome 21 markers SOD1, CBS, ETS2, D21S11, and BCEI showed partial trisomy 
by duplication of a chromosome segment carrying the SOD1, CBS, and ETS2 loci and 
flanked by the BCEI and D21S11 loci, which are not duplicated. This 
submicroscopic duplication at the interface of 21q21 and 21q22.1 reduces to 
about 2000-3000 kb the critical segment the trisomy of which is responsible for 
the phenotype of trisomy 21.