Streptavidin is a 47KD protein that extracted from streptomyces. Just like avidin, streptavidin has very high affinity to biotin molecule, one million times than the common affinity between antigen and antibody. Avidin is a alkalic protein (IP=10.0-10.5), and it can transfer to be a neutral protein through reconstruction. Isoelectric point of the streptavidin is near to neutrality, IP=6.0~6.5. Thus, streptavidin has very low non-specific absorption to tissue and cell. On the basis of streptavidin, the background of immunohistochemistry analysis is extremely low. With the method of polymeric labeling, streptavidin-HRP can form a complex which composed by about one hundred HRP and fifty streptavidin. And lots of enzymes can ensure the streptavidin-HRP with high sensitivity.