to what extent this linearity of response is present here. In this case it is the response of the total amount of all forms of TfR1 and TfR2 bound to Tf-Fe against the amount of Tf-Fe\_intercell that is important. A variable was created in the model to reflect the total receptor response (see Material and Methods), and this variable was followed in a time course response to an iron pulse (Figure \[fig:EpoIron\]A). The response to the iron pulse is remarkably similar to the response of the EPO receptor to EPO [@Becker2010Covering].

Becker et al. [@Becker2010Covering] reported that the linearity of EPO-R response, [*i.e.*]{} the integral of the response curve, is increased by increasing turnover rate of the receptor and this property was also observed in the simulation of TfR1 response (Figure \[fig:TfRResponse\]). The range in which the iron response is linear is smaller than that found for EPO (Figure \[fig:TfRResponse\]). As TfR1’s half life in the model matches the experimentally determined value [@Chloupkova2010Stoichiometries] the non-linear receptor response seen in the simulation is expected to be accurate. This suggests that TfR1 is a poor sensor for high levels of intercellular iron. On the other hand TfR2 is more abundant than TfR1 