C4.4A is known to be present in the placenta, skin, esophagus and peripheral mononuclear cells in humans. It is a GPI (glycosylphosphatidylinoditol) binding protein which has a similar structure to urokinase receptor (uPAR) and belongs to Ly-6 family as uPAR. While most of Ly-6 family molecules are consisted of single domain, uPAR is of three cysteine-rich domains. C4.4A is a membrane protein which binds to cell membrane in GPI binding site, and is consisted of two cysteine-rich domains and a cysteine lacking third domain. C4.4A was isolated from metastatic rat pancreatic cancer cell lines in 1989, and its human homolog was isolated in 2001. In human malignant tumors, it has been reported that C4.4A is expressed in malignant melanoma, urothelial, lung, esophageal and colon cancer. It is considered that C4.4A mediates invasion and metastasis of cancer cells by binding with laminin and degrading extracellular matrix in the progress of cancer. Therefore, immunological detection of C4.4A in cancerous tissue is expected to be an indicator of tumor metastasis.