The molecular basis of double cortex syndrome was investigated in 2 male patients. Magnetic resonance imaging of the patients' heads showed diffuse subcortical band heterotopia, as is seen in female patients. We found a heterozygous mutation for Asp50Lys or Arg39Stop in both patients. Microsatellite polymorphism analysis revealed that both patients had inherited a single X chromosome from their mothers. Restriction enzyme analysis using DNA extracted from the hair roots of each patient showed four different patterns in the combination of cells carrying wild and mutant alleles, which strongly suggest somatic mosaicism. We conclude that somatic mosaic mutations in the doublecortin gene in male patients can cause subcortical band heterotopia, and that molecular analysis using hair roots is a useful method for detecting somatic mosaicism.