Analyzing mammary gland carcinomas of different dog breeds (German Shepherd and Yorkshire Terrier) and mixed–breed dogs, some differences in VEGF-B, FLT1, ERBB2, GRB2, RAC1, CDH1 and HYAL-1 genes expression have been found, possibly showing that some breeds could have a limiting possibility to be used as a good BC model and while analyzing gene expression of canine mammary carcinomas, attention should be given to dog breed as a possible confounding factor. The gene discussed is ERBB2; the disease is breast cancer.