From the Paper "Mouse salivary androgen-binding protein (ABP), found in the saliva of the house mouse, or Mus musculus, has been found to have an interesting polymorphism. The alpha subunit of ABP has been found to be monomorphic for each of three subspecies of Mus musculus (domesticus, musculus, and castaneous), designating each subspecies with a different allele of ABPa (ABPa a, ABPa b, and ABPa c, respectively). ABP is thought to have evolved separately as Mus musculus radiated from the origin near the Punjab river valley about 500,000 years ago. Structural studies of this protein have shown that ABP is a dimer composed to two subunits, alpha and beta. Variations in the amino acid and DNA sequences of the alpha subunit have been found to exist between subspecies, but hardly any variation exists within the subspecies, reinforcing the idea that ABP is diagnostic of the subspecies. Behavioral studies involving mice and ABP have determined that female mice chose male saliva of their own ABP type significantly more frequently than male saliva of another ABP type. Therefore, ABP possibly acts as a pheromone and may play some role in mate and/or subspecies recognition in Mus musculus. If ABP's function has been determined correctly, then ABP is contributing to the speciation of Mus musculus, despite close contact of subspecies at the hybrid zones, where subspecies come into contact again after divergence from the origin."
