RANK is constitutively expressed in mammary epithelial tissues. Mammary duct ectasia can mimic breast cancer. AREG is a critical factor in estrogen action and ductal development of the mammary glands. Mammary glands do not contain muscle tissue. It aids in normal mammary gland development. Mammary glands develop during different growth cycles. Degraded basement membrane fragments (laminin-5) roles to lead the way of mammary epithelial cells migration. The tumors found were in either the liver or mammary glands of the animals. HGF is also importantly involved in normal mammary gland development. The development of the mammary gland occurs mainly after birth. Although possessing mammary glands, the platypus lacks teats. Many sources assert that mammary glands are modified sweat glands. Various studies in animal models of melanoma, mammary, and colorectal tumors have substantiated this hypothesis. The majority of deaths were caused by neoplasms, mainly mammary tumors. The number and positioning of mammary glands varies widely in different mammals.