Because the DMD gene is located at Xp21.2, which is one breakpoint of the inv(X), and because its defects are rarely associated with severe mental retardation, the other clinical features of this patient were deemed likely to be associated with the opposite breakpoint at Xq22.
Complete skipping of exon 66 due to novel mutations of the dystrophin gene was identified in two Japanese families of Duchenne muscular dystrophy with severe mental retardation.