Knowing that ERBB2 and TOP2α genes are located in the same chromosome in human and cat genomes, we analyzed the DNA copy number of ERBB2 (Figure 1a, Supplementary Table 1) and TOP2α genes (Figure 1b, Supplementary Table 2) in a collection of feline mammary tumors (n=27), always in comparison with the disease-free tissue from the same individual. This evidence concerns the gene ERBB2 and breast cancer.