Prions are infectious particles composed exclusively of protein and can cause a number of neurodegenerative diseases. Prions are coded for by a normal chromosomal gene which is found in the genomes of all mammals. A posttranslational event, as yet unidentified, converts the normal prion protein into a pathogenic isoform. Incubation time in prion diseases is controlled by either the prion gene itself or a scrapie incubation time gene closely linked to the prion gene. Gene cloning techniques were used to attempt to demonstrate the existence of the incubation time gene. DNA of two control mice, B6.I and I/lnJ, was compared to DNA from congenics with B6.I and I/lnJ parents. B6.I mice are known to have a shorter incubation time than the I/lnJ mice. Both mice have different alleles of the prion gene. Analysis of the DNA surrounding the prion gene was accomplished by using the polymerase chain reaction. Two markers showed polymorphisms between DNA of a control mouse and the DNA of a congenic. A polymorphism in these markers would allow the PCR to show the parental origin, either B6.I or I/lnJ, of the flanking DNA. A difference in incubation time of these mice from their parents shows that an incubation time gene, rather than the prion gene, is responsible for incubation time.