The hamster black tremor (bt) mutation induces a black coat color and a defective myelination in the central nervous system (CNS) that manifests as a tremor. On the other hand, loss-of-function mutations of the Attractin (Atrn) gene, such as Atrnmg, Atrnmg-L, and Atrnmg-3J in mice, and Atrnzi in rats, induce both darkening of coat color and hypomyelination and vacuolation in the CNS. The close resemblance of the mutant phenotypes led us to postulate that the bt/bt hamster also might harbor a mutation in Atrn. Here, we cloned the hamster Atrn cDNA and identified bt as a loss-of-function mutation of Atrn. While the human and rat Atrn genes encode both membrane- and secreted-type proteins, the hamster Atrn gene encoded only membrane-type protein with 1,427 amino acids, as in the case of the mouse. Hamster Attractin protein had 93.6%, 96.8%, and 96.8% identities with human, rat, and mouse membrane-type Attractin. In the brain of the bt/bt hamster, aberrant transcripts with more than three size species were observed, and the most predominant transcript encoded the truncated Attractin without transmembrane domain. In the Atrn gene of bt/bt hamster, an approximately 10-kb DNA fragment, which had 557-bp direct repeats in both ends and was flanked by the identical 6-bp target duplication sequences, was inserted into exon 24. In addition, the insertion was cosegregated with neurodegeneration in the CNS of 50 intercross progeny. These results indicated that the hamster bt mutation was the approximately 10-kb retrotransposon-like insertion into the Atrn gene, which resulted in aberrant transcripts. The bt/bt hamster will provide a useful tool for further understanding of the pleiotropic functions of Attractin.