paracrine growth factors, including igf-1, igf-2, amphiregulin, egf, fgf, hepatocyte growth factor (hgf), tumor necrosis factor α (tnf-α), tumor necrosis factor β (tnf-β), transforming growth factor α (tgf-α), transforming growth factor β (tgf-β), heregulin, wnt, rankl, and leukemia inhibitory factor (lif). these factors regulate cellular growth, proliferation, and differentiation via activation of intracellular signaling cascades that control cell function, such as erk, akt, jnk, and jak/stat. based on research with epidermal growth factor receptor (egfr) knockout mice, the egfr, which is the molecular target of egf, tgf-α, amphiregulin, and heregulin, has, similarly to the insulin-like growth factor-1 receptor (igf-1r), been found to be essential for mammary gland development. estrogen and progesterone mediate ductal development mainly through induction of amphiregulin expression, and thus downstream egfr activation. accordingly, erα, amphiregulin, and egfr knockout mice copy each other phenotypically in regards to their effects on ductal development. also in accordance, treatment of mice with amphiregulin or other egfr ligands like tgf-α or heregulin induces ductal and lobuloalveolar development in the mouse mammary gland, actions that occur even in the absence of estrogen and progesterone. as both the igf-1r and the egfr are independently essential for mammary gland development, and as combined application of igf-1 and egf, through their respective receptors, has been found to synergistically stimulate the growth of human breast epithelial cells, these growth factor systems appear to work together in mediating breast development. elevated levels of hgf and, to a lesser extent, igf-1 (by 5.4-fold and 1.8-fold, respectively), in breast stromal tissue, have been found in macromastia, a very rare condition of extremely and excessively large breast size. exposure of macromastic breast stromal tissue to non-macromastic breast epithelial tissue was found to cause increased alveolar morphogenesis and epithelial proliferation in the latter. a neutralizing antibody for hgf, but not for igf-1 or egf, was found to attenuate the proliferation of breast epithelial tissue caused by exposure to macromastic breast stromal cells, potentially directly implicating hgf in the breast growth and enlargement seen in macromastia. also, a genome-wide association study has highly implicated hgf and its receptor, c-met, in breast cancer aggressiveness. lactation upon parturition (childbirth), estrogen and progesterone rapidly drop to very low levels, with progesterone levels being undetectable. conversely, prolactin levels remain elevated. as estrogen and progesterone block prolactin-induced lactogenesis by suppressing prolactin receptor (prlr) expression in breast tissue, their sudden absence results in the commencement of milk production and lactation by prolactin. expression of the prlr in breast tissue may increase by as much as 20-fold when estrogen and progesterone levels drop upon childbirth. with suckling from the infant, prolactin and oxytocin are secreted and mediate milk production and letdown, respectively. prolactin suppresses the secretion of lh and fsh, which in turn results in continued low levels of estrogen and progesterone, and temporary amenorrhea (absence of menstrual cycles) occurs. in the absence of regular, episodic suckling, which keeps prolactin concentrations high, levels of prolactin will quickly drop, the menstrual cycle will resume and hence normal estrogen and progesterone levels will return, and lactation will cease (that is, until next parturition, or until induced lactation (i.e., with a galactogogue), occurs). breast size and cancer risk some factors of breast morphology, including their density, are clearly implicated in breast cancer. while breast size is moderately heritable, the relationship between breast size and cancer is uncertain. the genetic variants influencing breast size have not been identified. through genome-wide association studies, a variety of genetic polymorphisms have been linked to breast size. some of these include rs7816345 near znf703 (zinc finger protein 703); rs4849887 and rs17625845 flanking inhbb (inhibin βb); rs12173570 near esr1 (erα); rs7089814 in znf365 (zinc finger protein 365); rs12371778 near pthlh (parathyroid hormone-like hormone); rs62314947 near areg (amphiregulin); as well as rs10086016 at 8p11.23 (which is in complete linkage disequilibrium with rs7816345) and rs5995871 at 22q13 (contains the mkl1 gene, which has been found to modulate the transcriptional activity of erα). many of these polymorphisms are also associated with the risk of developing breast cancer, revealing a potential positive association between breast size and breast cancer risk. however, conversely, some polymorphisms show a negative association between breast size and breast cancer risk. in any case, a meta-analysis concluded that breast size and risk of breast cancer are indeed importantly related. circulating igf-1 levels are positively associated with breast volume in women. in addition, the absence of the common 19-repeat allele in the igf1 gene is also positively associated with breast volume in women, as well as with high igf-1 levels during oral contraceptive use and with lessening of the normal age-associated decline in circulating igf-1 concentrations in women. there is great variation in the prevalence of the igf1 19-repeat allele between ethnic groups, and its absence has been reported to be highest among african-american women. genetic variations in the androgen receptor (ar) have been linked to both breast volume (as well as body mass index) and breast cancer aggressiveness. cox-2 expression has been positively associated with breast volume and inflammation in breast tissue, as well as with breast cancer risk and prognosis. rare mutations women with cais, who are completely insensitive to the ar-mediated actions of androgens, have, as a group, above-average sized breasts. this is true despite the fact that they simultaneously have relatively low levels of estrogen, which demonstrates the powerful suppressant effect of androgens on estrogen-mediated breast development. aromatase excess syndrome, an extremely rare condition characterized by marked hyperestrogenism, is associated with precocious breast development and macromastia in females and similarly precocious gynecomastia (women's breasts) in males. in complete androgen insensitivity syndrome, a condition in which the ar is defective and insensitive to androgens, there is full breast development with breast volumes that are in fact above average in spite of relatively low levels of estrogen (50 pg/ml estradiol). in aromatase deficiency, a form of hypoestrogenism in which aromatase is defective and cannot synthesize estrogen, and in complete estrogen insensitivity syndrome, a condition in which erα is defective and insensitive to estrogen, breast development is completely absent.