A small supernumerary marker chromosome X identified by in situ hybridization.

Cytogenetic analysis of a girl with moderate mental retardation and dysmorphic features revealed a 46,XX/47,XX,+mar karyotype. Fluorescence in situ hybridization using chromosome specific alpha satellite probes showed that the supernumerary marker originated from the X chromosome. To our knowledge, this is the first reported case of a female patient mosaic for a supernumerary small marker chromosome derived from X, and hence mosaic for trisomy of the pericentric region of the X chromosome.