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In this family study of the narcoleptic syndrome, 52% of the probands had an affected first degree relative, 41.9% of the sibs of those probands with an affected parent were similarly affected, 33.3% of the parents of 2 affected sibs and 41.2% of the children (after a correction for age) had narcolepsy, cataplexy, or both.
Baraitser et al. (Tue,) studied this question.