We report on a family showing transmission of the fra(X) gene by 3 nonpenetrant, fra(X) negative, normally intelligent, full and half-brothers to their affected grandsons. The mothers of the affected boys are obligate carriers, fra(X) negative, and of normal intelligence. This family illustrates the "Sherman Paradox" and is compatible with the predictions of the Laird X-inactivation imprinting model. In addition, molecular and/or cytogenetic studies have enabled at-risk relatives to learn more about their carrier fra(X) status and have allowed for more accurate genetic counselling.