A full-length cDNA clone for GTP cyclohydrolase I (EC 3.5.4.16) was isolated from a mouse brain cDNA library by plaque hybridization. The nucleotide sequence determination revealed that the length of the cDNA insert was 994 base pairs. The coding region encoded a protein of 241 amino acid residues with a calculated molecular mass of 27,014 daltons. The deduced amino acid sequence of mouse GTP cyclohydrolase I was found to be highly homologous to rat (96%) and human type 1 (89%) enzymes.