Two brothers with mental retardation discordant for the Fragile-X syndrome

We describe two male sibs with mental retardation discordant for the fragile‐X syndrome. In the younger sib, chromosome analysis under folate deprivation showed a fragile site at Xq27.3 in 12‐46% of mitoses. In the older sib, however, repeated chromosome analyses (six different cultures with analysis of 50 mitoses each) under identical conditions could not detect any fragile‐X site. Using DNA probes linked to the fragile‐X gene, we found evidence that the two sibs inherited a different maternal X chromosome at Xq27.3. This excluded the presence of the fragile‐X syndrome in the older sib with a probability of greater than 99%.