We present a new Japanese family with hereditary progressive dystonia with marked diurnal fluctuation/dopa-responsive dystonia. The affected daughter and her asymptomatic father are heterozygous for a novel missense mutation that replaces His by Pro at codon 144 in the GTP cyclohydrolase I gene. Quantitative reverse transcription-polymerase chain reaction revealed a higher ratio of mutant/normal mRNA encoding GTP cyclohydrolase I in the patient. These results demonstrate the importance of mutant mRNA levels for phenotypic variability among cases with the same mutation.