Molecular and cytogenetic characterisation of an unusual case of partial trisomy/partial monosomy 13 mosaicism: 46,XX,r(13)(p11q14)/46,XX,der(13)t(13;13)(q10;q14).

A female infant with multiple malformations and mental retardation was noted to have a rare de novo chromosome abnormality involving mosaicism with two cell lines, one with a ring chromosome 13, and the other with partial trisomy 13 owing to a complex rearrangement. Cytogenetic examination excluded the presence of a t(13q;13q) cell line and showed a cell line with a marker chromosome containing two chromosome 13 long arms joined together after deletion of a part (q11-->q14) of one of them. In addition, the absence of a cell line with two normal chromosomes 13 or a cell line with a t(13q;13q) implies that the ring (13) and the marker (13) arose from a single event at the first cleavage division. The two cell lines were present in different proportions in both peripheral blood lymphocytes and skin fibroblasts. The results of microsatellite characterisation clearly indicate the paternal origin and the absence of recombination, supporting the postzygotic origin of both the ring and the marker chromosome.