Genomic DNAs were extracted from leukocytes of peripheral blood from the patients. Exons 2, 3, 4, and 5 of the RDH5 gene were amplified by polymerase chain reaction (PCR) on a thermal cycler (DNA Thermal Cycler 9700; Applied Biosystems, Inc., Foster City, CA). Primers were purchased from Life Technologies Oriental, Inc. (Tokyo, Japan). Approximately 200 ng of genomic DNA were amplified in a 50-μL reaction. The PCR products were purified with a kit (High Pure; Roche Molecular Biochemicals GmbH, Mannheim, Germany) and then directly sequenced with a DNA sequencing kit (Big Dye Terminator Cycle Sequencing Ready Reaction Kit, Applied Biosystems, Inc.), and an automated DNA sequencer (model 373; Applied Biosystems, Inc.). Primers for the sequence reactions were the same as those for the PCR reactions.