For some compounds and types of toxic effect there will clearly be a dose below which no effect or response is measurable. There is thus a threshold dose. This can be clearly demonstrated for quantal responses such as lethality, the presence or absence of a pathological lesion or a teratogenic effect for example. This means that there will be a dose at which the response dose not occur in any individuals in the population (Figure 1.7). Alternatively the concept could apply to a variable response such as enzyme inhibition which can be measured with increasing concentrations of the compound in question.