Men with Klinefelter's syndrome have fathered children using a version of the ICSI (intra-cytoplasmic sperm injection) fertility technique. Klinefelter's is a condition in which, due to the presence of an extra X chromosome, symptoms sometimes include mental retardation, but always signal infertility.

Men with Klinefelter's cannot produce viable sperm. A team of fertility specialists, led by Zev Rosenwaks at Cornell University, US, has bypassed this problem in a number of men by extracting immature sperm - spermatids - from their testes and, after choosing the 'best' ones, injecting them directly into an egg taken from their partner.

Reporting at a conference in Montreal last month, Rosenwaks said that of 15 patients with the syndrome treated in this way, nine achieved a pregnancy with their partner and went on to have children. All of the 14 babies produced had only one X chromosome, despite concerns that the ICSI procedure could allow genetic causes of infertility to be inherited by offspring.