"Different degree of mosaicism: That is the percentage of cells in your body that have the extra chromosome. Even when they do a blood test and tell you that "100% of the cells are XXY", what they are saying is "100% of the cells from your blood that we tested have XXY". They may have missed some cells without an X chromosome, and they do not know if the cells in your brain, in your skin, in your gut etc. etc. have all XXY. The concept is that there are men with 100% XXY in all their cells, and some others that have a variable percentage."
