Mosaicism is the presence of chromosomally normal and abnormal cells in the same individual. The percentage of mosaicism is known to vary in different tissues. We can easily study cells derived from the ectoderm, endoderm and mesoderm by the FISH technique, on buccal, urine and blood cells. When >5% aneuploidy for the X chromosome is detected in the follicular fluid cells derived from the ovary, we check for aneuploidy in these other tissues as well.