Abstract
While most recent studies have focused on the MDR1 gene and other similar types of multidrug resistance, two other phenomena (inhibition of the apoptotic pathway and re-growth resistance) have the potential for producing a much broader type of resistance to cytotoxic therapy. This speculative review discusses the potential contribution of these types of resistance and the possible interrelationships between the various types of resistance to cytotoxic therapy. The need for new approaches to assess the effects of biological agents is discussed.