Fig. (3).

The nt230(del4) MDR1 mutation in dogs. The canine MDR1 gene is organised in 27 exons, located on chromosome 14. The transcribed MDR1 coding sequence comprises 3846 bp and codes for the 1281 amino acids canine P-gp. The nt230(del4) mutation was localised to exon 4 and involves a 4-bp deletion followed by a premature stop codon. The truncated P-gp expressed from the mutant MDR1(-) allele is non-functional. Dogs with a homozygous nt230(del4) mutation are extremely sensitive to macrocyclic lactones.