Our data are consistent with findings that endogenous DUSP4 levels vary between melanoma cell lines in vitro.51 Relative overexpression of DUSP4 and ETV4 in SK-MEL-23 compared with CHL-1 may reflect the fact that SK-MEL-23 cells harbour amplified wild-type BRAF.52 Both cell lines used in this study harbour wild-type NRAS; given that ~60% of patients in the clinical DOC-MEK study had NRAS mutant melanoma, it may be informative to assess the contribution of DUSP4 and ETV4 to MEK inhibitor response in NRAS mutant cell lines. This evidence concerns the gene BRAF and melanoma.