in a gradient $\bar{g} = 550$ nM/mm of the cytokine CCL21, on top of a background concentration of $\bar{c} = 1100$ nM. Given a typical cell size of $a = 20$ $\mu$m, this corresponds to a percent difference of $\bar{p} = a\bar{g}/\bar{c} = 1\%$, similar to *Dictyostelium*. Yet, this also corresponds to a maximal molecule number difference of $\Delta\bar{n} = a^4\bar{g} = 53,$$000$, which is much higher than that of *Dictyostelium* (Table \[sense\_table\]). Even though the sensitivities are similar in terms of percent change, they are very different in terms of absolute molecule number.

Lower molecule numbers correspond to higher relative error. We can see this explicitly by writing Eq. \[eq:g\] in terms of the percent change $\bar{p} = a\bar{g}/\bar{c}$. Defining $\epsilon = \sigma_g/\bar{g}$ and taking $s \sim a$, we have $\epsilon \sim 1/\sqrt{\bar{p}^2a\bar{c}DT}$. Accounting for the fact that tumor cells (TC) have roughly twice the diameter as *Dictyostelium* cells (DC), this expression implies that the sensitivities of the two cell types over the same integration time $T$ to chemoattractants with the same diffusion constant $D$ satisfy $\epsilon_{\rm DC}/\epsilon_{\rm TC} = \sqrt{2\bar{c}_{\rm TC}/\bar{c}_{\rm DC}} \approx 18$. We see that because the *Dictyostelium* experiments were performed at lower background concentration, corresponding 