metallothioneins are a group of low - molecular - weight intracellular proteins present in high levels in fetal mammalian livers , bound to zinc and copper . they are also present in two major isoforms in low basal levels in various organs of adults in several species . 
 although a number of functions have been proposed for metallothioneins , their major biological roles may be in the storage of zinc and copper during rapid growth and development , and also in the detoxification of certain toxic metals . in adult liver , 
 metallothionein is mainly localized in the cytoplasm , it is localized also in the hepatocyte nuclei in human fetal liver and fetal and neonatal rat liver , as determined by immunohistochemical staining with a specific metallothionein antibody . because of its high expression in fetal development , the potential role of metallothioneins in human tumors was investigated . 
 the cellular localization of metallothionein was demonstrated in various human tumors such as thyroid tumors , testicular germ cell carcinoma , bladder transitional cell carcinomas , and salivary gland tumors . in most of these tumor tissues , metallothioneins were found in high levels in nucleus and cytoplasm in both benign and malignant tumors , although the proliferating edge of the malignant tumors showed most intense metallothionein staining . 
 the expression of metallothionein is not universal to all tumor growth ; its presence may depend on various factors , such as the type of tumor , cellular origin , morphological heterogeneity , or stage of growth . 
 human testicular seminomas , which are well differentiated , showed little expression of metallothionein irrespective of the staging , as compared to less well - differentiated embryonal carcinomas.(abstract truncated at 250 words)imagesfigure 3.figure 4 .