Different clinical features in monozygotic twins: a case of 7q--syndrome

We present male monozygotic twins who showed quite different clinical features. Blood chromosome analysis revealed 46,XY/46,XY,del(7) (q32-->qter) mosaicism in both twins. However, cultured skin fibroblasts from the twins showed different karyotypes. Twin 1, with a normal phenotype, had normal chromosomes and was 46,XY. Twin 2, on the other hand, had the characteristic manifestations of 7q- syndrome and chromosomes of 46,XY,del(7) (q32-->qter). DNA fingerprint analysis of their peripheral blood samples revealed the same pattern. However, DNA fingerprint patterns of cultured skin fibroblasts and buccal mucosal cells were different when a 7q terminal marker, probe g3, was used. These identical twins with discordant phenotypes can be explained by the occurrence of twinning and simultaneous erroneous mitosis. In addition, there might be a vascular communication which probably resulted in blood exchange and chromosomal mosaicism of the lymphocytes of the monozygotic twins.