The present invention provides a liver X receptor .alpha. splicing mutant protein which is an isoform of a liver X receptor .alpha., and which comprises at least an amino acid sequence encoded by exon 5 of a liver X receptor .alpha. gene, a gene thereof, and use thereof, and these can be utilized in development of methods and agents useful for preventing, diagnosing or treating diseases and abnormalities involved in inhibition of normal cholesterol metabolism by a normal liver X receptor .alpha..