| XYYY syndrome | |
|---|---|
| Other names | 48,XYYY |
 | |
| Karyotype of XYYY syndrome | |
| Specialty | Medical genetics  |
| Symptoms | Infertility, behavioural issues, developmental delays |
| Usual onset | Conception |
| Duration | Lifelong |
| Causes | Aneuploidy |
| Diagnostic method | Karyotype |
XYYY syndrome, also known as 48,XYYY, is a chromosomal disorder in which a male has two extra copies of the Y chromosome. The syndrome is exceptionally rare, with only twelve recorded cases. The presentation of the syndrome is heterogeneous, but appears to be more severe than its counterpart XYY syndrome. Common traits include borderline to mild intellectual disability, infertility, radioulnar synostosis (the fusion of the long bones in the forearm), and in some cases tall stature.