CD97 (EGF 1,2,5) is a member of the EGF-TM7 family of class Ⅱ seven-transmembrane (7TM) receptors,it has three isoforms according to different adhesive capacity with CD55 (also known as decay accelerating factor;DAF)protecting host cells from complement attack.In order to demonstrate the relationship between the expression of CD97 isoforms and its ligand CD55 and the clinicopathological features of colorectal carcinoma,56 colorectal carcinoma and their corresponding margins and normal specimens were investigated by RT-PCR,Western blot and immunohistochemistry.As a result,we found that transcript levels of CD97EGF(1,2,5) and CD55 were higher in the margin than those in tumors and normal epithelial mucous tissues (P<0.05).And the expression levels of CD97EGF(1,2,5) and CD55 mRNA had correlation with the clinicopathological features of colorectal carcinoma patients(P<0.05).All three groups of specimens were immunoreactive for CD97EGF(1,2,5) and the CD55 protein.Strong and specific immunoreactivities of CD97EGF(1,2,5) were located in the mucosal epithelia of the marginal basal membrane.Expression of CD97EGF(1,2,5) in the margins showed a marked difference among the depth of tumor invasion (T1 and T2,T3,T4 and T5),stages (Ⅰ and Ⅱ/Ⅲ/Ⅳ),Lymph node metastasis(with and without) and differentiation(low,high) of colorectal carcinoma (P<0.05).The expression of CD55 protein was highly correlated with CD97EGF(1,2,5) (R=0.7632,p<0.001).Western blot analysis confirmed the expression and distribution patterns of CD97EGF(1,2,5) and CD55.Our findings indicate that CD97EGF(1,2,5) may play a role in the development,invasion,differentiation and metastasis of colorectal carcinoma by binding its ligand CD55.Detection of the CD97EGF(1,2,5) expression in the tumor margin could be referred to as the molecular edge of colorectal carcinoma,it may be set as potential targets for treatment.