This research study in infant males with Klinefelter syndrome (47,XXY) will learn more about body composition (muscle and fat) and male hormones and look at the effect of testosterone shots on body composition. We know that older boys and men with Klinefelter syndrome often have more fat compared to muscle than adults without Klinefelter syndrome, but we do not know if this difference is present at birth or develops over time. We will learn if body composition and motor skills are improved with testosterone treatment in infants with Klinefelter syndrome.