greater than the expected number of structures located on the apex of the mammary gland on the integument surface into which the lactiferous ducts open; normal female mice have five pairs of mammary glands and nipples (males have no nipples); two pairs of nipples are situated in the abdomino-inguinal region (inguinal group) and three pairs in the ventral thoracic region (pectoral or thoracic group) (Mammalian Phenotype Ontology, MP_0009723)
