TESTO: Testosterone Effects on Short-Term Outcomes in Infants With XXY

Inclusion Criteria: * Male infants with 47,XXY karyotype identified prenatally who are 4-12 weeks old (31 to 90 days of age). 47,XXY must be from a diagnostic test such as Chorionic Villus Sampling (CVS), amniocentesis, or post-natal blood/tissue. Non-invasive prenatal screening results alone will not be accepted. Exclusion Criteria: * \>20 percent mosaicism for a normal cell line * Gestational age at birth \<36 weeks * Birth weight \<2.5th percentile or \>97.5 percentile for age (small or large for gestational age) * History of thrombosis in self or a first degree relative * Exposure to androgen therapy outside the study protocol * Use of medications known to affect body composition, such as growth hormone or insulin * Known allergy to the testosterone cypionate solution components including benzyl benzoate, benzyl alcohol, or cottonseed oil