Two DZ monoplacental twins are reported, one affected by mongolism and the other normal. Chromosome examination in the former indicated trisomy 21, while the latter presented a mosaicism consisting, at the age of 13 days, in 70% normal and 30% 21-trisomic cells; at the age of 5 months, in 95% normal and 5% trisomic cells; while, at the age of one year, all the cells appeared to be normal.
The possibility of mosaicism, and, according to clinical and hematological findings, that of feto-fetal blood transfusion are excluded.
The hypothesis is suggested of a progressive loss of the reproductive capacity of « chimaera » of abnormal lymphocytes.