To start exploring the potential of peptidyl arginine deiminase enzyme (PAD) inhibitors as anti-cancer drugs to treat canine and feline mammary cancer, the tumoral cell lines REM134 (canine) and K12–72.1 (feline) were incubated with increasing doses of BB-Cl-Amidine (BB-CLA) for 48 h and evaluated for viability using MTT assays. This evidence concerns the gene PADI4 and cancer.