Atypical Down syndrome and partial trisomy 21.

A case of "atypical" Down Syndrome (DS), where the proposita did not exhibit all of the clinical features of DS and had de novo partial trisomy 21, was studied. Results from phenotypic, chromosome banding and superoxide dismutase (SOD) gene dosage studies suggest a karyotype of 46,XX,-12,+t(12pter to 12qter::21q21 to 21q22.?2). Additional studies of such atypical cases will provide more precise sublocalization for both gene and phenotypic mapping of the bands that are responsible for the DS phenotype.