Copper toxicosis is an autosomal recessive disorder affecting Bedlington terriers, characterized by elevated liver copper levels and early death of affected dogs. Genetic linkage mapping studies initially identified linkage between the disease and the microsatellite marker C04107. Subsequently, the deletion of exon 2 of the copper metabolism domain containing 1 (COMMD1) gene (formerly MURR1) was shown to be the major cause of copper toxicosis, although the deletion breakpoints were not defined. In this investigation, polymerase chain reaction (PCR)-based techniques and sequencing were used to isolate the deletion breakpoints, utilizing the newly available dog genome sequence. The breakpoints were positioned at 65.3091 and 65.3489 Mb of dog chromosome 10, in intron 1 and intron 2 of COMMD1 respectively, a deletion of 39.7 kb. The two breakpoints share sequence homology suggesting that homologous recombination may have been responsible for the deletion. Using this information, a genomic diagnostic test for the COMMD1 deletion was developed and compared with microsatellite C04107 genotypes of 40 Bedlington terriers. Results from the 40 samples showed allele 2 of C04107 to be in linkage disequilibrium with the COMMD1 deletion.