To understand the genetic organization of the mouse sepiapterin reductase (mSR) gene, mouse genomic DNA fragments containing the mSR gene were cloned and exonic structure of mSR gene was characterized. When mouse strain 129 genomic library was screened with previously cloned probe DNA, 16 phage plaques which hybridized with the probe DNA from 400,000 independent plaques were isolated. Such an abundant presence of positive plaques suggested that mSR gene is not a single copy gene or might have many pseudogenes. Multiple band patterns of Southern blot data conformed it. By RNASPL program, possible exon-exon junction site were predicted. Several primers at the predicted site were designed to testify whether the sites contain intron. PCR and sequencing data reveal that the mSR genes have only one intron or limitted introns and have at least one pseudogene. It could be inferred that multi-gene family or multi pseudogene might be constructed by retrointegration or gene duplication of mSR gene.