Simple SummaryNiemann–Pick disease (NP) type C is an autosomal recessive metabolic disorder caused by mutations in theNPC1orNPC2gene. It is characterized by cholesterol accumulation in the lysosomes and late endosomes. Herein, we studied the molecular basis of Siamese and Japanese domestic (JD) cats that were previously diagnosed with NP. Sanger sequencing was performed on all exons of the two genes using genomic DNA extracted from the paraffin-embedded tissues of these cats. As a result, a missense mutation (NPC2:c.376G>A, p.V126M) was identified as a candidate pathogenic mutation in both types of cats. Several pathogenicity and stability predictors showed that this mutation was deleterious and severely decreased NPC2 protein stability. The Siamese cat was homozygous and the JD cat was heterozygous for this mutation. No other exonicNPC2mutations were detected in the JD cat; however, a homozygous splice variant (c.364-4C>T) was identified, which is not known to be associated with this disease.AbstractNiemann–Pick disease (NP) type C is an autosomal, recessive, and inherited neurovisceral genetic disorder characterized by the accumulation of unesterified cholesterol and glycolipids in cellular lysosomes and late endosomes, with a wide spectrum of clinical phenotypes. This study aimed to determine the molecular genetic alterations in two cases of felines with NP in Japan, a Siamese cat in 1989 and a Japanese domestic (JD) cat in 1998. Sanger sequencing was performed on 25 exons of the felineNPC1gene and 4 exons of the felineNPC2gene, using genomic DNA extracted from paraffin-embedded tissue specimens. The sequenced exons were compared with reference sequences retrieved from the GenBank database. The identified mutations and alterations were then analyzed using different prediction algorithms. No pathogenic mutations were found in felineNPC1; however, c.376G>A (p.V126M) was identified as a pathogenic mutation in theNPC2gene. The Siamese cat was found to be homozygous for this mutation. The JD cat was heterozygous for the same mutation, but no other exonicNPC2mutation was found. Furthermore, the JD cat had a homozygous splice variant (c.364-4C>T) in theNPC2gene, which is not known to be associated with this disease. TheNPC2:c.376G>A (p.V126M) mutation is the second reported pathogenic mutation in the felineNPC2gene that may be present in the Japanese cat population.