Klinefelter Syndrome (47, XXY) (Klinefelter's Syndrome) By Nina N Powell-Hamilton, MD, Sidney Kimmel Medical College at Thomas Jefferson University Reviewed / Revised Dec 2021 | Modified Sep 2022 View Patient Education Diagnosis Treatment Topic Resources Klinefelter Syndrome Klinefelter syndrome is the presence of two or more X chromosomes plus one Y, resulting in a phenotypic male.