The most common variations involve a trisomy, which means three sex chromosomes instead of the typical two. Girls who are born with an extra X chromosome are referred to as having Triple X or Trisomy X. Boys who are born with an extra X chromosome have 47,XXY, also known as Klinefelter syndrome. And boys who are born with an extra Y chromosome have 47,XYY, occasionally referred to as Jacob’s syndrome. In addition, there are a number of other X and/or Y conditions including 48 or 49 chromosomes. These include 48,XXXX, 48XXXY, 48XXYY and 48XYYY; and although increasingly rare, also 49XXXXX, 49XXXXY, 49XXXYY, 49XXYYY and 49XYYY. Some individuals may have two cell lines, which is called mosaicism, such as 46,XY/47,XXY.