Heller and Nelson expanded the characteristics of the syndrome to include variable degrees of Leydig cell dysfunction.  In 1956 Plunkett and Barr showed that patients with Klinefelter syndrome demonstrated a chromatin-positive buccal smear pattern, indicating an X chromosome variation was involved.  It was not until 1959, when Jacobs and Strong showed that the chromosomal pattern in these patients was XXY, that a clear understanding of the cause of Klinefelter syndrome occurred.

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