HSPA4 and melanoma: The around two-fold difference in the IC50 values estimated for PB in cell death assays (around 10–33 μM for A2058 and MeWo melanoma cell lines, respectively; Figure 4) and in vitro reactivation experiments (55–70 μM; Table 2), could be due to its possible action on different targets, such as calcium channels (see below) and the Hsp70 system, which might exert a synergistic effect that would result in an increase cell toxicity [71].