Read xxy for more information. Females have two X chromosomes (XX). Males have an X and a Y (XY). But in rare cases, a male is born with an extra X chromosome (XXY). This is Klinefelter syndrome. It’s also called Klinefelter’s or XXY. The primary features are infertility and small testicles. Often, men don’t even know they have Klinefelter until they run into problems trying to have a child. There’s no cure, but people with the syndrome can lead a regular life. With the right care, most men with Klinefelter lead normal, healthy lives. 
- An extra X chromosome in every cell, which is the most common
- An extra X chromosome in only some cells, called mosaic Klinefelter, where you don’t get as many symptoms
- More than one extra X chromosome, which is very rare and more severe
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