Klinefelter syndrome is the first human sex chromosomal abnormality to be 
reported. The majority of Klinefelter syndrome patients have the XXY karyotype. 
Approximately 15% of Klinefelter patients, however, are mosaics with variable 
phenotypes. Among the variant Klinefelter genotypes are such karyotypes as 
XY/XXY and XX/XXY. The variation in phenotypes most likely depends on the number 
of abnormal cells and their location in body tissues. In this paper we report 
the case of a 42-year-old patient with Klinefelter syndrome and a rare variant 
mosaic XXY/XX karyotype initially identified by GTG-banding. This was confirmed 
by fluorescence in situ hybridization (FISH) using a dual-color X/Y probe. The 
patient presented with erectile dysfunction and few other physical findings. 
Thus, this case illustrates a rare variant of Klinefelter syndrome with a 
relatively mild phenotype. It also illustrates the utility of FISH as an adjunct 
to conventional cytogenetics in assessing the chromosome copy number in each 
cell line of a mosaic. In our case, FISH also detected the presence of a small 
population of cells with the XY karyotype not previously detected in the initial 
30-cell GTG-banding analysis. Thus, through a combination of GTG-banding and 
FISH, the patient was determined to be an XXY/XX/XY mosaic. Given that most 
individuals with Klinefelter syndrome are infertile, and that these individuals 
may wish to reproduce with the aid of modern reproductive technology, such as 
testicular fine needle aspiration and intracytoplasmic sperm injection, it is 
important that accurate estimation of the frequency of abnormal cells be 
obtained for accurate risk estimation and genetic counseling, as recent studies 
in patients with mosaic Klinefelter syndrome revealed that germ cells with sex 
chromosomal abnormalities were nevertheless capable of completing meiosis.