A 27-year-old female having a baby with Down's syndrome was found to have a translocation t(14;21) and an acrocentric marker chromosome 15. The short arm of the marker was darkly stained in both G- and C-banded preparations, and had the normal Ag-NOR, but showed a homogeneously stained region in the distal part of the marker. By means of QM staining, the variant was found fluorescent, and Y-body like was found in about 9% interphase nuclei, which suggested that there might exist Y chromosome material in genomic DNA of proband. Dot blot hybridization using DNA probe pY3.4 derived from the heterochromatic region on the long arm of Y chromosome demonstrated that the proband carried Y chromosome material in her genome. Further, by in situ hybridization with the same probe, it showed that the variant of the marker had specific autoradiographic silver grains. So, the karyotype of the proband indicated that the combination of conventional banding analysis with chromosome-specific probe was of great significance in studying the micro-aberration of human chromosome.