studies of the toxic actions of 2,3,7,8-tetrachlorodibenzo - p - dioxin ( tcdd ) in numerous animal models and in human and animal cells in culture have established that the most characteristic pathologic lesions produced by this compound result from events initiated by the interaction of tcdd with a specific intracellular receptor protein , the ah receptor . 
 although most research on the interaction of tcdd with the ah receptor has focused on establishing involvement of this receptor complex in specific toxic responses , recent application of modern cell and molecular biology techniques is yielding new insights into the mechanism(s ) of signal transduction . 
 elucidation of these mechanisms is essential for understanding the molecular basis of the cell and species specificity which is a hallmark of tcdd toxicity . 
 this knowledge should provide the framework for development of a more toxicologically relevant risk assessment model.imagesfigure 1 .