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We report the first case of a 46,XY/47,XYY mosaic male with fragile X[Fra(X)] expression in both cell lines. Cytogenetic analysis, DNA linkage analysis, and direct detection of the pre‐ and full mutation for the affected individual and his at‐risk female relatives were performed. Southern analysis of PstI‐digested DNA with probe pX6 clearly distinguished the normal genotype, the premutation, and the full mutation in various individuals in the patient's family. Fra(X) carriers who had normal cytogenetic results were clearly identified by direct mutation analysis. © 1993 Wiley‐Liss, Inc.