Metadherin (Metastasis adhesion protein), also known as MTDH, LYsine-RIch CEACAM1 co-isolated (LYRIC), is a novel protein that localizes with the tight junction proteins ZO-1 and occludin in polarized epithelial cells. At the tight junction, it acts not as a structural component, but is rather recruited during the maturation of the tight junction complex. Metadherin is overexpressed in breast cancer tissue and breast tumor xenografts, while much lower levels are expressed in normal breast tissue. Metadherin binds to lung vasculature, one of the four common sites of breast cancer metastasis, through a C-terminal segment in the extracellular domain; blocking this lung-homing domain with antibodies or inhibiting metadherin with siRNA has been reported to inhibit breast cancer metastasis.