Virus-specific CTL derived from CB are of neonatal and not maternal origin. To confirm the origin of the virus-specific T-cell responses observed in the CB-derived CTL line CB3 were of neonatal and not maternal origin we used fluorescence in situ hybridization (FISH) with sex chromosome–specific probes. FISH probes specific to α satellite centromeric region of chromosome X labeled with spectrum orange and the satellite III (Yp12) region of chromosome Y labeled with spectrum green were obtained from Abbott Laboratories. Hybridization was performed according to the manufacturer's protocols. The slides were counterstained with 4,6-diamidino-2-phenylindole (DAPI) and the images were captured using 10×/25 aperture at magnification of 100×/1.40 oil R on Nikon E800 microscope equipped with a cooled-charge coupled device (CCD) camera. The cells were analyzed using Quips Pathvysion (Applied Imaging). A total of 200 interphase nuclei and 5 metaphase spreads were analyzed for sex chromosome signal pattern.