One of the primary sources of breast cancer is "terminal duct lobular unit involution". Involution is when mammary glands remove the milk-producing epithelial cells when they become redundant at weaning, resetting the glandular system, in a way. Involution also occurs throughout life as puberty, child-bearing and menopause change breast tissue structure.
But sometimes, involution may not occur frequently, or it may not be complete. Scientists know that lesser degrees of TDLU involution have been associated with increased risk of breast cancer, but they don't have a full understanding of what factors influence involution.
In this study, they looked at sex hormones' role in the involution process. Circulating hormones, such as progesterone, estradiol, testosterone and others, also are implicated in breast cancer risk. Could they also affect the involution process?