Background：

Laminins， a family of extracellular matrix glycoproteins， are the major noncollagenous constituent of basement membranes. They have been implicated in a wide variety of biological processes including cell adhesion， differentiation， migration， signaling， neurite outgrowth and metastasis. Laminins are composed of 3 non identical chains: laminin alpha， beta and gamma (formerly A， B1， and B2， respectively) and they form a cruciform structure consisting of 3 short arms， each formed by a different chain， and a long arm composed of all 3 chains. Each laminin chain is a multidomain protein encoded by a distinct gene. Several isoforms of each chain have been described. Different alpha， beta and gamma chain isomers combine to give rise to different heterotrimeric laminin isoforms which are designated by Arabic numerals in the order of their discovery， i.e. alpha1beta1gamma1 heterotrimer is laminin 1. The biological functions of the different chains and trimer molecules are largely unknown，