In humans, sex chromosomal abnormality is due to gene carried on X -chromosome. Increase in X-complement leads to Klinefelter's syndrome. Klinefelter's syndrome, 47, XXY or XXY syndrome is a condition caused by a chromosome aneuploidy. Affected males have an extra X sex chromosome. It is formed by the union of an XX egg and normal Y sperm or normal X egg and abnormal XY sperm. Affected males are almost always effectively sterile, although advanced reproductive assistance is sometimes possible and some degree of language learning impairment and mental retardation may be present. In adults, possible characteristics vary widely and include little to no signs of affectedness, a lanky, youthful build and facial appearance, or a rounded body type with some degree of gynecomastia (increased breast tissue).