1. Cassiman, J. J., Fryns, J. P., De Roover, J., Van den Berghe, H.: Sex chromatin and cytogenetic survey of 10,417 adult males and 857 children institutionalised in Belgian institutions for mentally retarded patients. Humangenetik 28, 43?48 (1975)
2. Court Brown, W. M.: Sex chromosome aneuploidy in man and its frequency with special reference to mental subnormality and criminal behaviour. Int. Rev. Exp. Path. 7, 31?97 (1969)
3. Dey, J.: Survey of 500 cases of Down's syndrome. Aust. J. ment. Ret. 1, 154?159 (1971)
4. Fujita, H. M., Fujita, K.: A cytogenetic survey a mentally retarded children. Jap. J. hum. Genet. 19, 175?176 (1974)
5. International Classification of Diseases (1967). Manual of the international statistical classification of diseases, injuries and causes of death. Eighth revision. World Health Organisation, Geneva