The analysis of the fine structure of the chromatids permits the identification of different regions on the long arm of chromosome 21. The preponderant role of the distal third of the long arm in the syndrome of trisomy 21 is now well established. Thus, trisomy of only band 21q22 results in a state identical to that caused by complete trisomy 21. If the trisomy involves only a part of band 21q22, the intensity of the symptoms is diminished, but the appearance of the patient is still reminiscent of Down's syndrome. Monosomy for band 21q22 results in a pathologic condition in which the morphological anomalies are the inverse of those observed in trisomic patients. This syndrome, as a "contre-type" to trisomy 21, is lethal. Trisomy of the proximal long arm region of chromosome 21 (21q21 leads to 21q22) is not associated with malformations but is accompanied by mental retardation. Monosomy of the same region results in a pathologic condition, which does not have features of the contre-type of trisomy 21.