[A family of hereditary motor and sensory neuropathy type 1B showing a marked difference of neurological disability score among affected family members].

The proband was a 22-year-old man with a complaint of progressive weakness in his lower limbs. His clinical diagnosis of hereditary motor and sensory neuropathy (HMSN) type 1 was made based on the neurological findings and the results of peripheral nerve conduction studies and of histological studies of the sural nerve obtained on biopsy. The molecular genetic studies revealed arginine-to-histidine substitution at amino acid 98 of the Po protein in the proband. Therefore, the final diagnosis of HMSN type 1B was made. The same mutation was found in his mother and his two sisters. The neurological disability score of the proband was 69. It was 24 for his affected mother. The scores were 16 and 20 for the two affected sisters. The reason why the proband showed the highest score was considered to be the presence of the marked muscle weakness in the lower limbs found only in the proband. Therefore, it was concluded that the neurological disability score differs greatly among the affected members even with the same genetic abnormality in the same generation of the same family with HMSN type 1B. The affected family member with a low neurological disability score may be clinically undiagnosed.