to lower absolute molecule numbers, the relative error in gradient sensing is $18$ times that of the tumor cells, despite the fact that both cell types are responsive to $1\%$ concentration gradients. Therefore, it is important to take note of the background concentration when studying the precision of gradient sensing. These data imply that *Dictyostelium* cells can sense noisier gradients than tumor cells. However, *Dictyostelium* cells have been studied more extensively than tumor cells as exemplars of gradient detection. It remains an interesting open question what is the minimum gradient that tumor cells can detect, not only in terms of percent concentration change, but also in terms of absolute molecule number differences.

We see that although cancerous cells and *Dictyostelium* cells are of similar size, their sensory responses to absolute molecule numbers can be very different (Table \[sense\_table\]). This difference is also reflected in their migration speeds: carcinoma and epithelial cells migrate [@wolf2003compensation; @wang2004differential; @gilles1999vimentin; @legrand1999airway] at $\sim 0.5 \mu\text{m}/\text{s}$ whereas *Dictyostelium* can migrate [@mccann2010cell; @song2006dictyostelium] at speeds of $\sim 10 \mu\text{m}/\text{s}$.

  ------------------------------------------ --------------------------- --------------------------------- -------------------------- ------------------------------
                                                                                                                                      
                                                   *Dictyostelium*                    Breast                Neurons [@rosoff2004new]             Mammary
                                              (Amoeba) [@van2007biased]   Cancer [@shields2007autologous]                              Epithelia [@ellison2016cell]
  Cell Length                                         10 $\mu$m                      20 $\mu$m                     10 $\mu$m                    10 $\mu$m
  Scale, $a$                                                                                                                          
  