In summary, all these data suggest that it seems unlikely that prednisolone induces DR through up-regulation of ABC-transporters (including P-gp), but it is not inconceivable that GC resistance through other mechanisms than upregulation of ABC-transporters, e.g., resistance to apoptosis, could coincide with DR to cytotoxic agents in dogs with lymphoma. This evidence concerns the gene ABCG2 and lymphoma.