Investigation of the expression of the amine oxidase family of proteins according to the molecular subtype of breast cancer revealed differences in the expression of stromal AOC3, MAO-A, and MAO-B; high expression of stromal AOC3 in luminal B and HER-2-type breast cancers; and high MAO-A expression in luminal A and luminal B (p < 0.001). The gene discussed is MAOA; the disease is breast cancer.