Table 2.

The connexins and their expression, localization, and function in normal breast and breast tumor.

Connexin	Normal breast		Breast tumor
	Expression	Function	Expression	Function
Cx26	Luminal epithelial cells	Luminal cell proliferation Production of milk	Downregulated in primary tumor Upregulated in metastatic tissue
Cx43	Myoepithelial cells Decreased during mid-pregnancy and lactation	Proper production and ejection of milk	Downregulated in primary tumor Upregulated in metastatic tissue	Increases invasion, adhesion of tumor cells
Cx30	Mouse luminal epithelial cells Peak expression at the onset of lactation	Lactation Compensates for impaired Cx32
Cx32	Mouse luminal epithelial cells Increased expression at parturition	Production of milk Compensates for impaired Cx30 function	Increased expression at metastatic lymph node
Cx46	Expression not reported in mouse or human normal breast		Expressed in primary tumor	May protect tumor cells from hypoxia induced death