46,X,i(Xq)/47,XX,+13 mosaicism.

A 10-year-old girl with short stature and other features of Turner's syndrome was found to be a mosaic consisting of 46,X,i(Xq) and 47,XX,+13 cell lines, a hitherto undescribed situation. She had none of the clinical features of trisomy 13 syndrome, with a possible exception of postaxial polydactyly of the left foot. Her PHA-stimulated blood lymphocytes and EB virus-transformed B lymphocytes both revealed the Xi(Xq)/XX,+13 mosaicism, while her skin fibroblasts showed an exclusively 46,X,i(Xq) karyotype. Studies using Q-and R-banding heteromorphisms as markers indicated that the patient started as a 13 trisomic zygote resulting from a maternal meiotic error, followed by the loss of chromosome 13 at an early mitotic division. C-banding analysis revealed two C banding blocks in the iso X chromosome, an indication that the chromosome was dicentric. BrdU-Hoechst-Giemsa analysis revealed that the iso X chromosome was late-replicating with both its arms either synchronously or asynchronously replicating. The iso X chromosome was thus designated as idic (Xq)(p11:p11). In view of the presence of the XX cell line, it was concluded that the patient started as an XX,+13 zygote, followed by two mitotic events, the loss of a chromosome 13 and the formation of the iso X chromosome, occurring either simultaneously or in succession.